Phosphinothricin tripeptide (PTT, phosphinothricylalanylalanine) is a natural-product antibiotic and potent herbicide that is produced by Streptomyces hygroscopicus ATCC 21705 (ref. 1) and Streptomyces viridochromogenes DSM 40736 (ref. 2). PTT has attracted widespread interest because of its commercial applications and unique phosphinic acid functional group. Despite intensive study since its discovery in 1972 (see ref. 3 for a comprehensive review), a number of steps early in the PTT biosynthetic pathway remain uncharacterized. Here we report a series of interdisciplinary experiments involving the construction of defined S. viridochromogenes mutants, chemical characterization of accumulated intermediates, and in vitro assay of selected enzymes to examine these critical steps in PTT biosynthesis. Our results indicate that early PTT biosynthesis involves a series of catalytic steps that to our knowledge has not been described so far, including a highly unusual reaction for carbon bond cleavage. In sum, we define a pathway for early PTT biosynthesis that is more complex than previously appreciated.