The DNA sequence of downstream and upstream regions of capsular polysaccharide biosynthesis (cps) gene cluster of Streptococcus agalactiae type la was determined. The overall structure of this region is very similar to a portion of the genome of S. pyogenes, although S. pyogenes does not contain cps genes. In the upstream region, a putative regulatory gene was identified and designated as cpsIaR. From the similarity of the amino acid sequences, the gene product seems to belong to an LysR family of transcriptional regulatory proteins. cpsIaR seems to be transcribed in the direction opposite to cps genes from two different but closely positioned promoters. GST-fused CpsIaR protein binds to the cpsIaA promoter region, suggesting that the protein controls the expression of cpsIa gene cluster.