A growing culture of Streptomyces lydicus converted biotin-(14)C to alpha-dehydrobiotin-(14)C. The conversion was demonstrated by isolating crystalline alpha-dehydrobiotin-(14)C from fermentation liquors supplemented with biotin-(14)C. The addition of pimelic acid-(14)C to the growing culture did not produce any radioactive alpha-dehydrobiotin. alpha-Dehydrobiotin did not substitute for biotin in Lactobacillus plantarum or in Saccharomyces cerevisiae. Antimicrobial activity of alpha-dehydrobiotin was abolished by avidin. alpha-Dehydrobiotin appears to be different from several biotin vitamers described in the literature. It is concluded that alpha-dehydrobiotin is a product of biotin catabolism in S. lydicus.