L-2-Amino-4-(2-aminoethoxy)-trans-3-butenoic acid was isolated from a fermentation broth of an unidentified species of Streptomyces, X-11,085. This substance inhibited the growth of Streptomyces cellulosae in a chemically defined minimal medium. The growth inhibition was reversed noncompetitively by L-cysteine and competitively by L-lysine, L-arginine and L-ornithine. An amino acid antimetabolite which inhibited the growth of Streptomyces cellulosae ATCC-3313 on a chemically defined medium was discovered in a fermentation broth of an unidentified Streptomyces sp. X-11,085 (deposited with the Northern Regional Research Laboratories under their number NRRL-5331). The growth inhibition was relieved by addition of either L-cysteine, L-arginine, L-lysine or L-ornithine to the medium. The antimetabolite was isolated by a combination of chromatographic techniques. Physical chemical characterization of the pure substance led to the structural proposal, L-2-amino-4-(2-aminoethoxy)-trans-3-butenoic acid (I).