A novel obligately intracellular bacterium, ileal symbiont intracellularis, which was obtained from the intestines of pigs with proliferative enteropathy disease, was grown in pure cocultures with tissue cultures of rat cells. An examination of the 16S ribosomal DNA gene sequence revealed that the isolates which we obtained are members of the delta subdivision of the Proteobacteria and that the sequences of these organisms exhibit a level of similarity of 91% with the sequence of Desulfovibrio desulfuricans ATCC 27774. These isolates were homogeneous and differed in cellular morphology, acid fastness, phenotype, electrophoretic protein profile, and habitat from Desulfovibrio species. On the basis of the results of an integrated study of the phenotype and genotype of a consistent morphological entity found in particular porcine cells and associated with a well-defined clinical condition, we concluded that these bacteria belong to a previously undescribed genus and species, for which we propose the name Lawsonia intracellularis gen. nov., sp. nov. A species-specific recombinant DNA probe was cloned previously, and this probe was used to identify the bacterium in tissue culture cells and in the ileal epithelia of pigs with proliferative enteropathy disease. Coculture of the organism with a rat enterocyte cell line allowed us to designate strain NCTC 12656 the type strain and to describe the new genus and species. The organism which we cultured is pathogenic for pigs and causes proliferative enteropathy lesions in their ilea and colons, and Koch's postulates were fulfilled for this organism.