Amino acid sequences of the small subunit of ribulose-1,5-bisphosphate carboxylase (SSU) of Acetabularia cliftonii and A. mediterranea were derived from five cDNA sequences of each of the two species of algae and by direct amino acid sequence determination of the isolated protein. An homology of more than 96% between the proteins indicates the close relationship between the two algae. All ten cDNAs in the reading frame display the termination codons TAA and/or TAG at various positions, which seem to code for the amino acid glutamine when compared with the amino acid sequence from the mature protein. This is reminiscent of proteins from ciliates where TAA and TAG also code for glutamine.