The transcriptional organization and the sequence of a 4000 base region of the chloroplast genome of Chlamydomonas reinhardii have been determined. The region includes the gene for the large subunit (LS) of ribulose bisphosphate carboxylase and portions of two neighbouring genes X and Y which are oriented in opposite direction relative to the LS gene. The transcripts of the LS, X and Y genes are 1600, 2200 and 2400 bases, respectively. The 5′ untranslated regions of the chloroplast messenger RNAs are variable in size: 91 ± 3 and 430 ± 10 bases for LS and X, respectively. The LS gene codes for a polypeptide of 475 amino acids whose sequence diverges 13 to 14% from the LS amino acid sequence of maize and spinach. The corresponding gene sequences differ 23 to 25% from each other. Most of the nucleotide differences occur in the third position of the codons and in the 3′ terminal portion of the gene. The three catalytic sites and the CO₂ activator region of the LS polypeptide have been highly conserved between C. reinhardii, spinach and maize. Only 40 different codons are used in the LS gene of C. reinhardii. The 5′ upstream regions of the LS and X genes contain sequences which resemble bacterial translational and transcriptional signals. Several repetitive elements are interspersed throughout the A + T-rich spacer between the X and LS genes. The 3′ untranslated region of the LS and Y mRNAs display stem-loop secondary structures which are reminiscent of bacterial terminators.