The distribution of Margaritifera margaritifera in southern Irish rivers and streams is described using records collected since 1987. Some 1800 sites on almost 400 rivers and streams were examined of which 8.1 and l4.6% respectively yielded shells. The mapping method employed, based on 10-km squares, distinguishes between sites with living specimens and empty valves. Although it was recorded in a wide range of sites, some analyses of the site characteristics indicate the following fluvial habitat preferences: sites within a distance of 5–40 km from source; sites with a channel width of 11–30 m; sites with non-calcareous soils and strata (though not a strictly calcifuge species); sites on lowland terrain; unpolluted sites with a shaded aspect. It is concluded that although M. margaritifera is widespread in the region and that its true distribution was hitherto poorly recorded, a decline in some of its former sites is nonetheless apparent.