As reported in our previous paper, 4-fluorothreonine, an antimetabolite of L-threonine, was found in the culture broth of the thienamycin producer Streptomyces cattleya. Thereafter, we isolated another antimetabolite in the course of purification of 4-fluorothreonine. This antibiotic was also an antimetabolite of L-threonine and was identified as β-ethynylserine from its physico-chemical properties. We report on the isolation, identification and biological properties of this antibiotic. We present the first report on production of β-ethynylserine by S. cattleya and on its antimetabolite activity against microorganisms.