Of a total of 124 Campylobacter (C.) strains isolated 1988 from fecal specimens, 4 (3.2%) were catalase-negative. Two strains were catalase-negative variants of C. jejuni. The other two were identified as C. upsaliensis on the basis of their biochemical properties and of the results of DNA-DNA hybridization with biotinylated probes from C. upsaliensis and C. jejuni. Both had been isolated from patients after their return from Asia (India and Thailand, respectively). In one case, a significant decrease of the antibody titre was observed within 12 months when the patient strain was used as an antigen. Since this patient had been suffering from diarrhoea during her stay and since with the exception of amoebic cysts, no other intestinal pathogen was isolated, C. upsaliensis is thought to have been the reason for the diarrhoeal episode.