The relationships of the fourteen (14) species of Campylobacter were derived by comparison of the partial 16S ribosomal RNA sequences determined using reverse transcriptase and oligonucleotide primers specific for regions in the 16S rRNA molecule. These species formed three distinct RNA sequence homology groups. We propose that the following species remain in the genus Campylobacter (homology group I): C. coli, C. jejuni, C. laridis, C. fetus, C. hyointestinalis, C. concisus, C. mucosalis, C. sputorum and the "catalase-negative to weakly-positive" (CNW) strains. C. pylori, "C. cinaedi" and "C. fennelliae'' were not related to the true campylobacters at the genus level. However, they were related to each other and to Wolinella succinogenes at the genus level and constituted homology group II. These four species should be reclassified and placed within a single genus based on 16S rRNA sequence similarity. C. cryaerophila and C. nitrofigilis also exhibited a high level of RNA sequence homology with each other but not with any other species tested; they constituted homology group III and should be considered as being a single genus. A comparison of the 165 rRNA sequence data from the three homology groups with the same sequences from representatives of the three major phylogenetic branches of the purple bacteria indicated that these genera form a single phylogenetic branch which is only distantly related to the purple bacteria. / Master of Science