Males possess a modified tubular anal fin called a gonopodium located directly behind the ventral fin which is flexed forward and used as a delivery mechanism for one or more balls of spermatozoa. The male will approach a female and will flex his gonopodium forward before thrusting it into her and ejecting these balls.

The Females anal fin is shaped like a fan.

Guppies, Platies, Swordtails and Mollies are all sexed the same way.

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