The vet told out new born kittens apart at a week old! And he was right. LOL.
But the males will have testicles LOL. Unless they have been snipped off... then they wont but you will still be able to tell and eventually the female will have nipples!
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II'm assuming these are kittens. contrasting the two can be helpfu lf you have both sexes. The space between the anus and urinary tract opening in the male is greater than the distance in the female.
I was always told a female kitten's genitals will resemble an upside-down exclamation point (the vaginal slit below is the anus). On a male kitten the organ will look more like a colon. The penis is below the anus and separated by a space to accomodate his testicles. You may even be able to see very small lumps in that space thatwill grow into the testicles as the kitten matures.
it depends on how young the cat is. if they're at least 4 months you should be able to tell. unless the boys are nuetered already....anyway the boys have balls. the girls have nothing.