In surveys of river and stream sites in Ireland 10 per cent yielded Theodoxus fluviatilis. The 176 records collected since 1980 have been used to delineate the fluvial distribution: the method employed plots negative and positive loci for the species as well as abundance of sites within 10-km squares. The occurrence was tested against the solid geology, natural chemistry and size of the sites to see the relationship: 94 per cent of occurrences were from sites overlying Carboniferous strata with the bulk of these, 88 per cent, on limestone; the natural chemical characteristics of the sites from which it was recorded had the following ranges pH 7.0–8.4, alkalinity 66–304 mg/l of CaCO3 and hardness 90–328 mg/l of CaCO3; it was found in all river/stream size categories sampled but exhibited an apparent preference for larger sites. It was recorded from some sites with minor saline intrusion and at one location with a salinity range of at least 0.18–9.6 parts per thousand. It was found at a few sites which were classed as slightly or moderately polluted. The records from the present study were combined with the post 1950 recordings of the Conchological Society to give the overall distribution for the species in canals, lakes, rivers and streams in Ireland. Although absent from the north, including Northern Ireland, and south-west, Theodoxus is widely spread in the country. Factors which might be important in limiting or facilitating distribution are briefly discussed.