We report the identification and characterization of the ste (Streptomyces eps) gene cluster of Streptomyces sp. 139 required for exopolysaccharide (EPS) biosynthesis. This report is the first genetic work on polysaccharide production in Streptomyces. To investigate the gene cluster involved in exopolysaccharide 139A biosynthesis, degenerate primers were designed to polymerase chain reaction amplify an internal fragment of the priming glycosyltransferase gene that catalyzes the first step in exopolysaccharide biosynthesis. Screening of a genomic library of Streptomyces sp. 139 with this polymerase chain reaction product as probe allowed the isolation of a ste gene cluster containing 22 open reading frames similar to polysaccharide biosynthesis genes of other bacterial species. Involvement of the ste gene cluster in exopolysaccharide biosynthesis was confirmed by disrupting the priming glycosyltransferase gene in Streptomyces sp. 139 to generate non-exopolysaccharide-producing mutants.