Between November 1990 and February 1991 101 gull faecal samples, collected in central Scotland, and 50 cloacal lavages, from gulls captured at two refuse tips near Durham, England were examined for the presence of Cryptosporidium sp. oocysts. Five of 101 (c 5%) of faecal samples and 11 of 50 (22%) of cloacal lavages contained oocysts, of which 64% and 83%, respectively were considered viable when examined with propidium iodide and 4'-6-diamidino-2-phenylindole. Since there is insufficient evidence to ascribe these oocysts to a recognized species they are therefore referred to as Cryptosporidium sp. oocysts. There were significant differences in the occurrence of oocysts between gulls captured at the different refuse tips (P < or = 0.01), but no significant difference between the distribution of oocysts in two species of gull, Larus argentatus (Herring Gull) and L. ridibundus (Black-head Gull). The differences may be explained by different food sources and feeding habits. The contribution of gulls to environmental contamination with Cryptosporidium sp. oocysts is probably generally small, but may be more significant when large numbers roost on surface waters.