A new allosamidin analog, termed didemethylallosamidin (3), was isolated from the mycelia of Streptomyces sp. AJ 9463 which is a producer of allosamidin (1) and demethylallosamidin (2). 14C-Labeled 1, 2 and 3 as well as their related compounds, 4, 5, and 6, were prepared to investigate the biosynthesis of 1. Conversion experiments with the labeled allosamidins revealed that 2 was a precursor of 1, but 3 was not incorporated. This suggests that the first N-methyl group is introduced before the cyclization of the aminooxazoline ring during the biosynthesis of 1. Although none of the compounds 4, 5, and 6 were converted to 1, the production of 1 was inhibited by the addition of 4.