During a 3-year period, "Campylobacter upsaliensis" was isolated from 99 patients. Phenotypic characterization and numerical analysis of protein electrophoregrams showed evidence that "C. upsaliensis" is a distinct Campylobacter species with unique characteristics. The MBCs of 13 antibiotics were determined. In general, these organisms were highly susceptible to drugs that were present in the selective isolation media, making none of the available selective media suitable for the isolation of "C. upsaliensis." Ten strains were found to be resistant to erythromycin (MBCs, .12.50 mg/liter). Plasmid DNA was detectable in 89 of the 99 strains; 16 plasmid profiles could be identified. Plasmid pattern 16, containing four plasmids of 52, 32, 5.5, and 2.6 megadaltons, represented 60.7% of the plasmid-containing strains. None of the "C. upsaliensis" strains could be agglutinated with antisera against heat-labile antigens from C. jejuni, C. coli, or C. laridis. "C. upsaliensis" was found to be susceptible to serum killing and was readily phagocytized by human polymorphonuclear cells. Catalase-negative or weakly positive (CNW) strains of Campylobacter were first isolated from dogs by Sandstedt et al. (31). In the same report, those investigators were able to demonstrate, by DNA hybridization, that these organisms belonged to a new species; the name "Campylobacter upsaliensis" was proposed in 1986 (K. Sandstedt and J. Ursing, Abstr. XIV Int. Congr. Microbiol. P.B8-17, p. 61, 1986). The species description was minimal, although a type strain (NCTC 11541 = LMG 8850) was designated. Neither the species name nor the type strain has standing in the literature, because the species description was not published in a peer-reviewed journal. CNW strains were isolated from five children with diarrhea by Mégraud and Bonnet in France (F. Mégraud and F. Bonnet, Letter, J. Infect. 12:275-276, 1986); Steele and McDermott (32) recovered eight CNW isolates from diarrheic stools of Australian children and one CNW isolate from an adult; Patton et al. (27) showed evidence that "C. upsaliensis" is responsible for enteritis in both adults and children in the United States; Taylor et al. (36) isolated "C. upsaliensis" from seven patients, both adults and children, with diarrhea in Canada; Lastovica et al. (16) isolated these organisms from blood cultures of pediatric patients in South Africa; Walmsley and Karmali (39) isolated "C. upsaliensis" from six children. Most recently, Fox et al. (8) isolated "C. upsaliensis" from three asymptomatic cats. In this report, we characterize and describe the "C. upsaliensis" group, using strains isolated in our hospital. We use the species name in quotation marks to indicate its proposed name and refer to the "type strain" as a reference strain.