The majority of capsular polysaccharides (K antigens) are linear molecules and their genes have a common functional organisation encoding common steps in capsule biogenesis. However, the K4 antigen is a substituted polymer composed of a chondroitin backbone with a fructose side chain. In order to determine whether K4 biosynthesis uses these common mechanisms the K4 antigen genes were cloned. DNA probes taken from the two conserved regions of the K1 genes were used to isolate one plasmid, pRD1, homologous to both probes. Immunological analysis was used to show that pRD1 directs the production of the substituted K4 antigen on the cell surface. Southern hybridisation was used to show that the cloned genes are organised in the same way as other K antigen gene clusters. We conclude that the branched K4 antigen is handled by the same post-polymerisation mechanisms as other linear K antigens.