ABSTRACTThe Badenoch Group of Experts on Cryptosporidium recommended that a survey should be made of river and borehole sources to assess the incidence of oocysts. This paper presents the results of a national survey of three river stretches and six boreholes carried out between January 1990 and March 1991. A total of 1754 samples were analysed and 266 (15%) of these were subjected to independent checks in reference laboratories. The results show a low incidence of oocysts in the sources (range 0–4 oocysts/1). One river stretch gave consistent positive results but with low concentrations (range 0.04–3 oocysts/1, n = 318). Parallel epidemiological studies showed a low incidence of cases in the community, none of which could be related specifically to water supplies. The concentrations of oocysts found are generally lower than those found in studies in the US particularly in comparison with published results for large industrialized rivers.