Instructions
Look for the gonopodium. This is a modified anal fin that is found only on males. It is a small, sharp-looking fin that is located on the male molly's belly.
Look at the belly of the fish. Female mollies have a gravid spot, a dark spot underneath their bellies. When the female molly is pregnant, the gravid spot becomes darker.
Examine the shape of the molly. The female is significantly more rounded than the male, which is more streamlined and sleek.
Look at the dorsal fin. The dorsal fin is the fin on the back of the molly, and in females it is shorter and blunter. Male mollies tend to have fins that are longer and rise smoothly from their backs.
Examine the size of the molly. Mollies grow to a full length of four inches, and females are significantly larger than males when full grown. While this is not a precise way of sexing mollies, it is helpful when looking at a group of them, where there are plenty of other specimens for comparison.
How to Identify Male and Female Mollies
Mollies are freshwater fish that originally come from the southern United States. They are live-bearing fish, meaning that they give birth to live young rather than lay eggs. Mollies are a hardy fish and simple to keep, and breeding them is as easy as leaving males and females in the same tank. To ensure successful breeding, learn to distinguish between male and female mollies.