Two surveys were carried out in the Corrib catchment (Ireland) to determine the physical and chemical factors that govern the distribution of Corixidae. Of the twenty one species recorded, five species, Sigara scotti (Fieber), S. distincta (Fieber), S. &sat-urn (Leach), S. fallenoidea (Hungerford) and Cymatia bonsdot$$i (Sablberg) comprised 82% of the numerical total. Although individual species occurred in chemically diverse sites species assemblages and changes in the relative abundance of the majority of species and in species richness and diversity were evident in the progression from hard to soft water. High altitude soft water sites had a much lower number of species compared with chemically similar low altitude sites. In Lough Corrib, the main body of water in the catchment, most species were abundant only in sheltered areas with mud substrates and high percentage vegetation cover. Species diversity was high in these areas and either C. bonsdorj$i, S. fossarum, S. fallenoidea or Callicorixa praeusta (Fieber) dominated numerically. However, S. scotti, S. dorsalis (Leach) and Arctocorisa germari (Fieber) were more abundant in exposed areas with sand, gravel or mearl substrates. In temporary ponds and in lotic water species diversity was also high but species composition (mainly C. praeusta, Sigura nigrolineata (Fieber), Corixapunctata (Illiger), C. panzeri (Fieber), C. bonsdorjj%) was different to that of temporally stable lentic habitats.