The distribution patterns of four species of the genus Idotea in the Limfjord (inner Danish waters) are described. The species show habitat segregation with respect to the degree of exposure and to some extent with respect to the salinity. Idotea pelagica and I. granulosa are restricted to habitats with high exposure and high salinity, while /. chelipes only occurs at sheltered habitats with a relatively low salinity. The fourth species, I. baltica, is found on moderately exposed localities. The findings are compared with data from other regions and discussed in terms of interspecific competition and habitat selection.