Guppy Gender

Michael Langerman
4 min readMar 12, 2020

This is a pair of 15 month old guppies.
Usually, an adult female guppy is (underlined in red) larger and bulkier than a male guppy (underlined in blue).

The same couple of guppies at 7 months.

Males usually sport brighter colors and have fancier shaped tails.

Here, you can see another male with two females.

All three fish are 6 months old.

Notice that in this example, both females are about the same size or smaller than the male.

Look at these two (about 5 months old) guppies.
One of them is a male with a fully developed underbelly (anal) fin, as only males have.
It is a tube with the scientific name gonopodium.

The shape of the anal fin is the only reliable way to visually distinguish males from females.

All newborn guppies look alike, resembling females.
Here is a dozen of 5 day old fry.

August 15th, 2017
Here are three nursery aquariums with guppy babies.
Their birthday is indicated on top of each aquarium.

These are about 3 days old guppy fry.
All newborns look alike.
The visible sex differences develop in following weeks and months.

Here are 5 weeks old guppies.
Some fish grow faster than others.

Notice the largest one looks like an adult female.

Here are 2 month old guppies of different sizes and all look like females.

Male guppies mature sooner than females do.
This is a 6 week old guppy male.

His anal fin gets the male shape in the last week.

Looking on a fish from the side is the easiest way to see the shape of the anal fin.

It looks like a stick for a male guppy.

Females have a fan shaped anal fin.

Watch this male flex his anal fin.

Often, it’s easier to see the shape of the anal fin by looking at the shadow of the fish.

While generally, guppy males mature a couple month before females, occasionally Nature plays tricks, and some males mature later than females do!

Keep an eye on your fish’s anal fins!

Have fun and happy fish :)