Clostridium botulinum 62-A was shown to catabolize l-arginine via citrulline to ornithine, NH(3), and CO(2). The individual enzymes of the dihydrolase system were all demonstrated in extracts of cells, spores, and germinated spores. There was no liberation of urea from l-arginine, so no functional arginase enzyme is present, but there was some transamidinase activity in cell extracts. l-Ornithine was degraded at a significant rate by cells grown in an l-ornithine-supplemented medium; it was partially decarboxylated to putrescine and partially fermented to NH(3), CO(2), volatile acids, and delta-aminovaleric acid. Results from the fermentation of l-ornithine-C(14), -1-C(14), and -2-C(14) demonstrated that essentially all of the CO(2) was derived from carbon 1, and volatile acids from carbons 2 to 5. Assays for the products of l-ornithine-C(14) fermentation revealed that the volatile acids consisted of acetate, propionate, valerate, and butyrate (in order of decreasing concentrations), and that delta-amino-valerate was the primary reduced product. A small amount of citrulline was formed during the fermentation. The carbon and redox balances indicated that l-ornithine is fermented as a single substrate. Preliminary experiments demonstrated that the fermentation of l-ornithine is carried out by cell extracts with the production of volatile acids.