The nature and frequency of men's and women's sexual fantasies were investigated by surveying 307 students (182 females, 125 males) at a California state university or junior college via a paper‐and‐pencil questionnaire. The questionnnaire was inspired by modern evolutionary theory and was designed to investigate sex differences in sexual fantasies. Substantial sex differences were found in the salience of visual images, touching, context, personalization, emotion, partner variety, partner response, fantasizer response, and inward versus outward focus. These data, the scientific literature on sexual fantasy, the historically‐stable contrasts between male‐oriented pornography and female‐oriented romance novels, the ethnographic record of human sexuality, and the ineluctable implications of an evolutionary perspective on our species, taken together, imply the existence of profound sex differences in sexual psychologies.