Group B streptococci, Streptococcus agalactiae, produce high-molecular-weight polysaccharides containing N-acetylneuraminic acid. Although the type-specific capsular polysaccharide (CP) synthesis (cps) genes of several S. agalactiae strains have been extensively analyzed, to date, no sialyltransferase activity has been detected from any gene product of the cps gene cluster. Among the cps genes, the cpsK gene products of S. agalactiae types la and Ib showed weak similarity to several bacterial sialyltransferases. In this study, the cpsIaK and cpsIbK gene products were found to show sialyltransferase activity specific for lacto-N-neotetraose and lacto-N-tetraose, respectively. This acceptor specificity seems to reflect the respective CP structure, since the repeating unit of type la CP is sialyllacto-N-neotetraose and that of type Ib CP is sialyllacto-Ntetraose. We also found that the C-terminal regions of CpsKs were almost completely conserved in various S. agalactiae strains.