Abstract All-female parthenogenetic species afford a unique opportunity to probe the nature and origin of sexual behavior. Whereas the majority of vertebrates are gonochoristic (that is, they rely for reproduction on fertilization of female ova by male sperm), at least 27 species of squamate reptiles representing seven families consist mostly or entirely of female individuals and produce clones of all-female offspring. Male-like (pseudomale) behavior patterns have been observed in five parthenogenetic lizard species that are remarkably similar to the courtship and copulatory behavior of closely related gonochoristic species. This behavior is directed only toward preovulatory females and can be elicited by administration of exogenous androgens. While this pseudomale courtship and copulatory behavior is not obligatory or essential for reproduction, it does have a facilitatory effect, increasing a female's fecundity. These experiments demonstrate that the presence and behavior of conspecifics serve as a neuroendocrine primer in all-female parthenogenetic lizards, as does male sexual behavior in sexually reproducing vertebrates. These findings, and other evidence, also suggest that sexual behavior antedated and was a prerequisite for the evolution of gonadal sex.