- The most obvious difference between a male and female molly is the shape of its anal fin. Male livebearers have a specialized anal fin called a gonopodium; this fin is elongated and sword-shaped. Females have rounded, delta-shaped anal fins.
- Female mollies are generally larger than males. Adult male mollies rarely exceed 3 inches in length, but females may grow to 4 inches or longer. Other factors such as age, water quality and tank size may affect a molly's adult size.
- Adult female mollies almost always carry fertilized eggs or embryos; as a result, their midsections are noticeably rounded. Males have a slim, streamlined appearance.
- A molly's dorsal (back) fin may vary depending on its sex. A female molly's dorsal fin is usually shorter and less impressive than a male's. These differences are not universal.
- Male mollies constantly display courtship behavior. They may chase females, display their gonopodia and attempt spawning embraces to gain the female's attention. When females are absent, males may display these behaviors to each other or to members of other species.
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