A strain of Streptomyces libani NK-110134 (FERM 10638) has been found to produce 4-thiouridine (Fig. 1), which exhibits inhibitory activity against Bacillus subtilis and causes complete regression of HeLa S3 cells. Chemical synthesis of 4-thiouridine was reported in 1959¹) and microbial ribosylation of 4-thiouracil in 1977²). The nucleoside is present as a minor component of tRNA in Escherichia coli³). We are reporting an efficient method for microbial production of 4-thiouridine as an alternative to chemical synthesis.