Three cDNA clones coding for the contact site A (csA) protein, a cell adhesion molecule of Dictyostelium discoideum, were isolated by screening a cDNA library with monoclonal antibodies. Two of these clones contained the complete coding region for the csA protein of 1542 bp including a sequence of 57 bp coding for the leader. The N terminus of the mature protein, as it was published previously, was identified in the amino acid sequence derived from both full-length cDNA clones. Southern blot analysis suggests the presence of only one csA gene in the haploid genome. Accumulation of the csA-specific message of 1.9 kb begins during development on nitrocellulose filters at 9 h of starvation, and reaches a maximum at 12 h, the time of cell aggregation. Expression of the csA glycoprotein follows closely accumulation of the transcripts. In the multicellular slug stage following cell aggregation, the amount of csA transcripts rapidly declines to low levels.