An antagonist of phenylalanine, L-l, 4-cyclohexadiene-l-alanine, was isolated from the culture broth of a strain of Streptomyces designated Streptomyces diastatochromogenes var. sakaii. It is active against plant-pathogenic fungi, and Pseudomonas aeruginosa on synthetic medium. Producing strain, and the production, isolation, identification and biological properties of this antibiotic are described. In the course of a search for microbial metabolite which would be protective against infection of rice by Piricularia oryzae and Ophiobolus miyabeanus, an antibiotic was isolated from the culture filtrate of a strain of Streptomyces. This antibiotic was found to be an antagonist of phenylalanine and identified as L-l,4-cyclohexadiene-lalanine from physico-chemical properties and chromatographic behavior. Although L-l, 4-cyclohexadiene-l-alanine was chemically synthesized by Snow et al. in 1957, its biological production has not been reported.