Both theoretical and empirical studies have treated mate-guarding in aquatic Crustacea purely as a male decision problem. However, male and female interests are rarely identical, as implied by observations of female resistance against guarding attempts. We tested experimentally the occurrence of sexual conflict over guarding duration in three crustacean species: Idotea baltica, Asellus aquaticus (Isopoda), and Gammarus zaddachi (Amphipoda). Specifically, we manipulated, by osmotic stress or a neuromuscular blocking agent, the female's ability to resist guarding attempts. Female manipulation, by both methods, roughly doubled precopula duration in I. baltica (Figs. 1 and2) showing that female resistance effectively diminishes guarding duration. However, in A. aquaticus and G. zaddachi female manipulation had no effect on guarding duration, which also was longer than in I. baltica (Fig. 2). This implies either that male and female interests are equal or that the conflict is resolved according to the male interest in these species. The lack of female resistance in such species allows long precopulatory guarding. In I. baltica we also manipulated, by osmotic stress and by clipping nails, male ability to hold the female. These treatments had no effect on guarding duration (Figs. 1 and2). Male size tended to correlate positively with guarding duration in control groups, but not in female manipulation groups (Fig. 3). Thus, conflict is mainly resolved according to the female interest in/. baltica. Results in this species also suggest that female resistance selects