In laboratory experiments females of an aquatic isopod Idotea baltica preferred the undersides of stones and Mytilus shells while males preferred the light-green upperparts of bladderwrack (Fucus sp.). Within the sexes, habitat selection was not clearly related to the colour morph. It is suggested that sexual differences in habitat selection may affect the susceptibility of colour morphs to visually searching predators and so contribute to the evolution of sexual dimorphism in colour morph frequencies in I. baltica. The activity of males remained high during the entire breeding period from May to July, whereas females became inactive just after the deposition of eggs into the brood pouch. Females were more active at night than during the day throughout the summer, but the activity of males during the day increased to the level of that at night by July. This may lead to greater mortality of males in late summer. The reduced activity of gravid females may be a behavioural adaptation to an increased risk of predation during the incubation period.