During the course of our screening program for new bioactive compounds, we have isolated a cytotoxic metabolite UK-1 from the acetone extracts of Streptomyces sp. 517-021}. UK-1 (Fig. 1) is structurally unique bis- (benzoxazole) in which the 2-position of one benzoxazole is joined to the 4-position of a second benzoxazole r-ng2,3) UK-1 did not show any antimicrobial activity, but its alkaline hydrolysate, demethyl UK-1 (DMUK-1, Fig. 1), was active against some bacteria. This paper presents the antibacterial action of DMUK-1.