A suspected waterborne outbreak of presumed campylobacteriosis involving approximately 680 of the 1000 inhabitants in a community is described. Twenty-two strains of Campylobacter jejuni were isolated from patients. Ten randomly chosen isolates were identified as biotype 1 LAU 0:1 (n = 9) and biotype 2 PEN 0:6, 7 (n = 1) and one from tap water as biotype 1 PEN 0:19, 22. The majority of cases occurred within a period of 1 week. Examination of human sera obtained during the first 2 weeks of the epidemic (n = 38) revealed two patients producing antibodies directed only against the water isolate. The majority (89%) of sera collected 4 weeks later (n = 18) had antibodies directed against the most common human isolate. We suggest that all three serotypes of C. jejuni were involved in this outbreak, stressing the need for continuous surveillance and treatment of public drinking water sources.