We investigated the occurrence of thermotolerant Campylobacter and Yersinia spp. in three surface water sources in Norway which represented different levels of pollution and eutrophication. Samples were collected every fortnight during a 14-month period. In addition, samples from 100 private wells were examined for campylobacters only. Campylobacter was recovered from 42 (43.8%) of the 96 samples of surface water, whereas Yersinia spp. were isolated from four (4.2%) of the samples. Campylobacter was not isolated from the well water samples. The highest isolation rate of Campylobacter was obtained from the two most polluted water sources. The proportion of positive samples was significantly higher in the autumn (71.4%) than in the spring (36.4%) or summer (22.2%). The highest overall isolation rate was obtained at water temperatures ranging from 2.1 to 8.0 degrees C, and the lowest at temperatures greater than 15 degrees C. Logistic regression analysis showed a highly significant relationship between the prevalence of Campylobacter and the number of three types of indicator bacteria: faecal coliforms, faecal streptococci and sulphite-reducing clostridia. Of the 60 Campylobacter isolates obtained, 51.7% belonged to C. jejuni biotype 1, 20.0% belonged to C. jejuni biotype 2, 21.7% to C. coli, 3.3% to C. lari and 3.3% were non-typable. All four Yersinia isolates were non-pathogenic variants.