The Staphylococcus epidermidis genes icaABC are involved in the synthesis of the polysaccharide intercellular adhesin (PIA), which is located mainly on the cell surface, as shown by immunofluorescence studies with PIA‐specific antiserum. PIA was shown to be a linear β‐1,6‐linked glucosaminoglycan composed of at least 130 2‐deoxy‐2‐amino‐D‐glucopyrano‐syl residues of which 80–85% are N‐acetylated, the rest being non‐N‐acetylated and positively charged. A transposon insertion in the icaABC gene cluster (ica, intercellular adhesion) led to the loss of several traits, such as the ability to form a biofilm on a polystyrene surface, cell aggregation, and PIA production. The mutant could be complemented by transformation with the IcaABC‐carrying plasmid pCN27. Transfer of pCN27 into the heterologous host Staphylococcus carnosus led to the formation of large cell aggregates, the formation of a biofilm on a glass surface, and PIA expression. The nucleotide sequence of icaABC suggests that the three genes are organized in an operon and that they are co‐transcribed from the mapped ica A promoter. Ica A contains four potential transmembrane helices, indicative of a membrane location. The deduced Ica A sequence shows similarity to those of polysaccharide‐polymerizing enzymes, the most pronounced being with a Rhizobium meliloti N‐acetylglucosaminyltransferase involved in lipo‐chitin biosynthesis (22.5% overall identity and 37.4% overall similarity). This similarity suggests that Ica A has N‐acetylglucosaminyltransferase activity in the formation