Species that are sexually monomorphic show no difference between the two sexes other than their genitalia. They have no secondary sex differences, and no real differences in size, colour, eating habits, etc. This phenomenon is the opposite of sexual dimorphism, where there are marked differences between the sexes.

One example of a sexually monomorphic organism is the New World Monkey (Platyrrhini). These silly little monkeys live in South and Central America, and have long, prehensile (grasping) tails.

Anthropologists have speculated that organisms who tend to live in mixed sex groups (rather than harems where one male is dominant or as solitary adults) tend to be more likely to show sexual monomorphism, whereas organisms living in a society where competition for females in much more fierce tend to show a greater degree of sexual dimorphism.