Adult male sea lice Lepeophtheirus salmonis were more likely to leave host fish Atlantic salmon Salmo salar if they detected the chemical cues of other adult male lice than if they detect cues of female lice. The detection of both male and female chemical cues yielded an intermediate response. These results suggest that males use chemical cues to balance competition for resources and mate acquisition, and they highlight the need for further studies of the chemical ecology of this important parasite.