The natural occurrence of plasmids in "Campylobacter upsaliensis" (catalase negative or weak campylobacters: CNW group) was investigated. The 62 strains isolated from patients with gastroenteritis or bacteraemia in five countries (South Africa, Belgium, Sweden, UK and Australia) were screened for plasmid DNA, which was detected in 54 (87%) of them. Plasmids were of various sizes between 1.4 and 90-Md and up to three or possibly four plasmids were present in 26 (42%) strains. A total of 15 different plasmid profile types were identified. Strains of profile type 2, containing 2.8-, 34- and 60-Md plasmids and a less distinct 5-Md plasmid, were the most frequently encountered (22 strains) and they exhibited a wide geographical distribution. Overall the 34- and 2.8-Md plasmids were the commonest occurring in 58 and 55% of plasmid containing strains respectively. Nine strains from dog faeces were also examined and some had identical profiles to human isolates, suggesting a possible animal to man transmission route. These findings show that plasmid profiling has considerable potential as a typing method for "C. upsaliensis".