Instructions
Look at the molly's anal fin, located on the underside of its belly. In female mollies, this fin is the shape of a rounded triangle. Males have a modified anal fin called a gonopodium; this pointed tube looks narrow and sword-like.
Observe the fish's body structure. Male mollies have a slim, streamlined appearance, and females tend to be rounder. A very rotund molly is almost certainly a female carrying a full-term pregnancy.
Wait a few weeks or months to observe potential signs of breeding. Males will frequently chase females and engage in spawning embraces. Any fish that has not given birth within several months in a new home is most likely male-- female mollies will almost inevitably give birth every 30-60 days. Even females who are not housed with males are usually impregnated before they are introduced to a new tank.
How to Tell Male or Female in Mollies
Livebearing fish like mollies are easy to sex. These brackish-water beauties are fairly easy to keep, but because mollies are prolific breeders, many beginning aquarists choose to keep only one sex of molly. Fortunately, all breeds and subspecies of molly have obvious differences between the male and female individuals of the species. Differences in anal fin shape, build and behavior can help beginning aquarists identify male and female mollies.