In a laboratory experiment in I. baltica the precopulatory guarding was preceded by a period of struggles between the sexes as males continuously tried to initiate the precopulatory guarding and females resisted their guarding attempts. This struggling lasted for a few days, during which the females escaped from the males on the average 1.3 times per hour. While the females resisted, the males usually responded by kicking back. Once the precopula started, on the average 43 h before the completion of the female parturial ecdysis, the female resistance stopped. If the guarding male was replaced by another male, the female accepted the new male without resistance or resisted only weakly. Larger males were able to perform longer precopulas, and furthermore, when males were hunger stressed they performed shorter precopulas than control males. The female resistance and the existence of struggles imply a conflict between the sexes over whether or not to start the precopulatory phase. This conflict may occur either because of different optimum precopula duration of the sexes or because of the unwillingness of the females to pair with whatever male. By resisting, females may, at least to some extent, control the duration of the guarding, and the resistance may lead to selection among male candidates. Thus the female resistance, although for so far largely neglected, may have potential importance in the mate choice and sexual selection of aquatic crustaceans with precopulatory guarding.