The protandrous marine snail Crepidula fornicata has been a theoretical and empirical model for studies of sex change for many decades. We investigated the social conditions under which sex change occurs in this species by manipulating physical and chemical contact with conspecifics. Male snails were either in physical and chemical contact with females or in chemical contact with, but physically isolated from, females. Males were tested both with living females and with empty, sterilized shells. Males that were physically touching a living female were less likely to change sex than the isolated controls, while males in chemical (but not physical) contact with females changed sex no slower than the isolated controls. These results provide experimental evidence that the factor controlling sex change in C. fornicata is due to a contact-borne inhibitor associated with female conspecifics. These findings serve as a basis for future studies of sex change in this model system.