Sexing Guppies

The key is in the anal fin.

In males, the fin right under the base of the tail is pointy and looks "squished together". In females, this fin is shaped like a normal fin, like a triangle or a fan.

Other than that, usually females have a big blackish area at the back of their abdomen towards their tail when they are pregnant, except on a dark-colored female.
 
Also in adults its easy. Males have bigger more colorful fins. Tail fin is much bigger. Thats assuming you are talking about fancy tail guppies. I know never assume.:p
 
The small ones chasing the big ones with an evil gleam in their eyes are the males.

The harrassed but resigned looking, drabber, big ones are the females.

:D
 
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