A physical map of the chloroplast DNA of Chlamydomonas reinhardii has been established which includes EcoRI, BamHI and BglII restriction fragments. The map is circular with a size of 190 kb. The two ribosanal RNA coding regions are contained within two 19.5 kb inverted repeats whose ends are 72 and 78 kb apart on the genome. DNA from a cloned 1.5 kb fragment, mapping adjacent to the ribosanal unit, can hybridize to most of the BamHI chloroplast restriction fragments, indicating the presence of repetitive sequences in the chloroplast genane. The 4S RNA coding regions are interspersed throughout the genome although not randomly. The gene for the large RudP-carboxylase subunit is located on the 5.6 kb EcoRI fragment Rl5 which is not adjacent to the restriction fragments carrying the ribosomal RNA genes. Preliminary evidence suggests that regions of the chloroplast DNA, which include also non ribosomal DNA fragments are preferentially transcribed and/or that the corresponding transcripts are stable.