L-2-Amino-4-methoxy-£nz?zs-3-butenoic acid was isolated from a fermentation broth of Pseudomonas aeruginosa ATCC-7700. This substance inhibited the growth of Bacillus species 1283B in a chemically defined medium. The growth inhibition was reversed by a variety of amino acids. An amino acid antimetabolite which inhibited the growth of Bacillus sp. 1283B was discovered in a fermentation broth of Pseudomonas aeruginosa ATCC-7700. The compound was isolated from the broth by adsorption onto an anion-exchange column followed by elution with trimethylammonium bicarbonate2), a volatile buffer. The structure was determined to be L-2-amino-4-methoxy-/r£ms-3-butenoic acid, I*, by physical chemical characterization and by reduction to L-2-amino-4-methoxybutanoic acid3), Ilia, identical with a sample prepared by enzymatic resolution of the synthetic racemate4), Illb. An unexpected product, L-2-amino-butanoic acid, II, was isolated on reduction of I employing a platinum-charcoal catalyst. The title compound is the first amino acid reported to contain an enol ether group.