Ribulose bisphosphate carboxylase-oxygenase (Rubisco) is a key enzyme in the photosynthetic fixation of CO₂ by the chloroplast. The synthesis of the enzyme is an example of the cooperation between the chloroplast and the nucleocytoplasmic compartments, as it is assembled from subunits encoded in the two respective genomes. I have used a synthetic oligonucleotide probe to isolate the nuclear Rubisco small subunit genes (rbcS) directly from a genomic library of Chlamydomonas reinhardtii DNA. They constitute only a small family: there are two rbcS genes, and an additional related sequence, in the C. reinhardtii genome. All three are clustered within 11kb at a single locus, and should thus be particularly well suited for genetic manipulation. The pattern of expression of rbcS RNA is dependent on the growth conditions.