One hundred and eighteen (118) Campylobacter strains were studied by DNA homology experiments and characterized phenotypically. These strains formed eleven (11) distinct DNA homology groups (species) corresponding to C. fetus, C. "hyointestinalis", C. jejuni, C. coli, C. laridis, C. nitrofigilis, C. sputorum, C. mucosalis, C. concisus, and two unnamed groups currently referred to as the aerotolerant campylobacters and the "catalase-negative or weak" (CNW) strains. For practical reasons, we propose retaining the subspecies fetus and venerealis designations for C. fetus. In addition, we propose that the subspecies sputorum and bubulus designations for C. sputorum be dropped and replaced with biovars sputorum, bubulus and fecalis, the latter biovar including the catalase-positive strains formerly known as C. “fecalis". Biotyping schemes are also presented for C. jejuni and C. coli. Growth at 25 and 42°C, sensitivity to nalidixic acid and cephalothin, growth in semisolid medium containing 1% glycine, 1% oxgall or 3.5% NaCl, growth in a semisolid minimal medium (MM), anaerobic growth in 0.1% trimethylamine-N-oxide (TMAO), H₂S production in Sulfide-Indole-Motility (SIM) medium, or on triple sugar iron (TSI) agar slants, hippurate hydrolysis, aerobic growth on agar plates, a requirement for H₂ or formate for microaerophilic growth or H₂ or formate and fumarate for anaerobic growth, alkaline phosphatase activity, and deoxyribonuclease (DNase) activity proved to be the most useful phenotypic characteristics for identifying these strains at the species, subspecies and biovar levels. / Ph. D.