We have isolated and characterized a full-length cDNA clone encoding the precursor of the small subunit (pSU) of ribulose-1,5-bisphosphate carboxylase/oxygenase (RUBISCO) from the green alga, Chlamydomonas moewusii. Comparison with the C. reinhardtii rbcS1 gene sequence reveals that both small subunit (SS) coding regions are 75% homologous and that their predicted mature polypeptide chains are each composed of 140 amino acids. In contrast, their transit peptides appear to be divergent. We also show that transcription of the C. moewusii rbcS gene(s) which generates a 1,230 and a 930 base mRNA species are light-stimulated/or accumulated during the light period of the cell cycle. Finally, the SS polypeptide sequences of fifteen different photosynthetic organisms are compared; this analysis reveals at least five well-conserved polypeptide domains.