Numerous porphyrin auxotrophic mutants have been isolated from the 168 trpC2 strain of Bacillus subtilis by selection with streptomycin. Some of them could be supplemented with ALA while the majority grew only in the presence of haemin. Among the latter strains, the syntropism test allowed to distinguish two groups different in phenotype, viz., feeders accumulating ALA and non-feeders accumulating instead of ALA other porphyrin intermediates. On the basis of transductional studies, feeders and non-feeders could be divided into two and four groups, respectively. Biochemical investigation revealed that, with one exception, one enzyme of the porphyrin biosynthesis was coordinated to each hem locus. The following genes were identified:hemB yields ALA-dehydrase;hemC yields PBG-deaminase; hemE yields uroporphyrinogen decarboxylases; hemF yields coproporphyrinogen oxidase; hemG yields protoporphyrin-iron-chelatase.