How to Identify Male and Female Guppy Fish Fry?

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Identifying the gender of your fish can sometimes be a challenge, especially when sexual dimorphism is not characteristic to a species.

This is not the case with guppy fish since the differences between adult male and female guppy fish are easily recognizable.

Of course, this cannot be said about guppy fry as well, which look the same regardless of gender, at least the do so at the beginning of their lives.

If you’re raising guppy fry commercially or if you want to increase the growth rate of guppy babies, this article in which I explain how to identify male and female guppy fry will be useful for you.

I also explain why you should separate baby guppies by gender and when is the best time to identify the sex of guppy fry.

Best Time to Sex Guppy Fry

When I first started to breed guppy fish, I had some trouble sexing fry too. But as I researched them more and observed guppy babies in their development, I started to spot the differences with much more ease.

If you’re distraught at first, hang in there, you’ll be able to notice the particularities of each gender quite easily after a few tries.

When they’re born, all guppy fry look the same regardless of gender. There’s simply no way of telling which is male and which is female.

As they continue to grow, differences start to appear. These differences are subtle in the beginning, but as growth rate accelerates, they become more and more noticeable.

Within 4 weeks or as soon as they reach at least half an inch in size, you’ll see that fry that all looked the same a few weeks ago, now start to have certain traits that set them apart.

Therefore, the best time to sex guppy fry is at around one month of age, since by then the subtle differences become more obvious and you’ll be able to confidently identify their gender.

In all fairness, if you’ve been watching them grow and develop, you’ll immediately notice differences even if you don’t really know what to look for.

Correctly determining the gender of your guppy fish and fry has multiple implications that are best understood early on, so you won’t end up with an aquarium overrun with guppies.

So, for starters, it’s important to tell the gender of your guppies from an aquarium stocking perspective. If you don’t want to have them breed, you must avoid putting both genders in the same aquarium.

You can set up an all-male aquarium, or an all-female aquarium, to prevent them from breeding out of control.

Even if you’re setting up a mixed-gender aquarium, it’s still important to know the male to female ratio, so you can keep them in the right numbers.

It’s not advisable to keep more than one male for 2-3 females because they’ll breed continuously, which of course means you’ll end up with hundreds of fry that you can or can’t accommodate.

Plus, continuously producing fry isn’t as healthy for your fish as it can shorten their lifespan. In this regard, it’s best if breeding frequency is controlled and happens on your terms.

Then there’s the issue of making your fish commercially available. Once again, you must be able to correctly identify the gender so you can sell your fry based on gender or as pairs, for example.

Why You Should Separate Guppy Fry by Sex?

I mentioned that correctly identifying the gender of guppy fry is important if you want to stock the aquarium with only one gender, if you want to sell them, or have them breeding.

But there’s another reason for it, which can be just as important, especially from a growth rate perspective.

Separating guppy babies by gender is a good way to increase the growth rate of guppy fry. That’s because as soon as they reach sexual maturity (at around 3 months), they’ll be able to breed.

Breeding will take up a lot of energy and potentially slow down they growth rate, which you want to avoid if you’re planning on selling them.

Besides separating them based on sex, there are other ways to accelerate the growth of guppy fry including offering them enough space, offering them a varied and high-quality diet, making sure their water is clean and well-maintained, etc.

While separating them by gender is important to promote their growth, it’s also true that this is not the only way to have them grow faster, and you should also strive to offer them the best conditions possible.

How to Tell the Gender of Guppy Fry?

At first you may struggle to tell male and female guppies apart, but as you get more experienced, it will become second nature to spot the differences.

When guppies are adults, things are much easier since the differences are rather remarkable — females are usually larger and longer, they have a bigger abdomen, and they aren’t as colorful.

Male guppies have more beautiful tails, are smaller and more colorful than females. They also have differing anal fins (male anal fins are longer and pointed, female anal fins are squared off and smaller).

Ok, but what about guppy fry?

Distinctive marks are a bit more difficult to ascertain when guppies are still in their initial growth phase, so don’t hesitate to use a strong flashlight or a magnifier to identify differences that can indicate the gender.

Look for the Gravid Spot in Females

Located at the base of the tail, the gravid spot signals the location where the fry will grow when the female guppy becomes pregnant.

This dark colored spot is specific to females and becomes more apparent as the female guppy gets older and older.

Beginners may not recognize it easily, but a professional breeder can identify the gravid spot in guppy fry very early on.

Look for a dark area near the abdomen and anal fin and if it’s a female guppy fry, you’ll be able to identify the gravid spot.

Identify the Gonopodium in Males

Although both genders have anal fins, in male guppies this is called a gonopodium, and it’s the reproductive organ used for fertilizing the female.

In livebearing fish like guppies, fertilization occurs inside the female’s body. Eggs are also hatched inside the body, allowing the female to give birth to live fry.

Guppies can breed monthly and drop hundreds of fry at a single time, which can easily create aquarium overstocking issues if you’re not prepared to manage it.

To correctly identify the anal fin in male guppy fry, it’s best if you wait 2-3 weeks, because at 1-2 weeks female and male guppy anal fins look exactly the same.

Once the gonopodium appears, you’ll be able to confirm without a shred of doubt the gender of the fry.

When male guppies reach sexual maturity, you’ll also notice them chasing the female guppies to reproduce, which can also be a signal that you can use to tell their gender apart.

Colorful Guppy Babies are Male

Color is another good feature to base your decision on when determining the sex of guppy fry. Color formation in male guppy babies starts within 3 weeks.

Females are less colorful, so any sign of color development on the body of guppy fry signals that they’re male. If faint streaks of color are limited to the tail only, then they’re most probably females.

Because males display more impressive colors compared to females, you may be inclined to keep an all-male aquarium.

This is a good idea if you don’t want to breed guppies as an all-female tank will still potentially produce some fry if one of the females has been fertilized or is already pregnant.

Do Female Guppy Fry Grow Faster Than Males?

You may notice that some guppy fry grow faster than others. Differential growth rates can be explained by various factors including differences in sex.

Females guppies can indeed grow faster than male guppies and even among males, guppy fry that get access to more food will grow faster.

Because larger guppy fry will better compete for food than smaller ones, it makes sense to separate them by their size too, so as to increase the growth rate in smaller fry, which would only be possible if there’s less competition for food.


Don’t worry if you’re struggling at first to distinguish the gender of guppy babies. It takes some practice to get it right. With time and experience, you’ll become better and better at this.

With some fish that don’t exhibit sexual dimorphism, you can’t rely on physical aspects alone to determine their gender. Luckily, in guppy fish gender is easy to recognize, albeit a bit later in their development process.

So, whether you’re raising guppy fry for commercial purposes or just as a hobby, it’s good to know how to determine the sex of guppy fry, so you can separate them by gender. This will not only help them grow faster, but it will be useful in controlling their population too.

Updated: August 21, 2022

Questions & Answers

  1. Pamela E Russell

    My guppies had babies about 6 wks ago. I still cant tell the gender. Could it be possible that they are all the same sex?

    1. avatar

      It is totally fine that you can’t tell the difference between males and females at 6 weeks old. If you grow them in an aquarium, they will develop much slower than in a pond. I think in about 2-4 weeks you will be able to tell which are males and females. Just have more patience.

    1. avatar

      Haha! 🙂 It is always a challenge for me as well to convince family members that one more tank is necessary. I just got “permission” for 5 more tanks 🙂

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