Random amplified polymorphic DNA (RAPD) analysis was used to study genetic variation among schistosome hosts of the species Biomphalaria pfeifferi. The analysed snails came from 15 sites along a 6-km stretch of a Zimbabwean river, providing data on genetic-geographical relationships over, probably, the smallest scale yet studied for any snail intermediate host species. Only 6% of the DNA fragments were common to all snails, and snail populations as geographically close as 100 m were genetically distinct. The most genetically polymorphic snail populations were those collected within and downstream from human water-contact sites.