Fifteen kilometres of a river system traversing rural and urban areas and subject to sewage works effluent discharge was studied during a 12 1/2 month period. A total of 312 samples was collected from 12 sites at 14 d intervals and tested by a glass microfibre filtration method and a most probable number (MPN) method. Campylobacters were found in 43% of samples by the filtration method and 21% by the MPN method. The lowest frequency of isolation and lowest counts (less than 10 campylobacters/100 ml) were associated with samples collected from rural sites and fast-flowing stretches of river. The greatest frequency of isolation and highest counts (greater than 10-230 campylobacters/100 ml) were associated with sites adjacent to or downstream of sewage works. There was an obvious seasonal trend; most isolations and highest counts were obtained in late autumn and winter, and fewest isolations and lowest counts in spring and summer. Surface water run-off from adjacent farmland following heavy rainfall also increased the counts of campylobacters in the river system. Biotyping of isolates demonstrated that the most prevalent Campylobacter sp. was Campylobacter jejuni but C. coli, C. laridis and a previously unrecognized group of campylobacters were also isolated. Serotyping differentiated 14 serotypes of C. jejuni, 11 of C. coli and two of C. laridis. Furthermore, serotypes of C. jejuni commonly isolated from enteritis in man were frequently found in river water tested during this study.