Plastid (pt) DNA from the red alga Porphyridium aerugineum was purified by CsCl gradient centrifugation. An EcoRI library of the ptDNA was screened with a gene probe specific for the gene encoding the large subunit (LSU) of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco EC 4.1.1.39) from spinach. A 5.8 kb EcoRI clone containing the LSU gene (rbcL) was isolated and the DNA sequence of the Porphyridium rbcL gene and its flanking regions was determined. An open reading frame was found 130 bp downstream from the rbcL gene that shows homology to genes coding for the small subunit of Rubisco (rbcS) from higher plants and cyanobacteria. Both genes (rbcL + rbcS) are cotranscribed. Comparison of rbcL and rbcS sequences from Porphyridium, higher plants and cyanobacteria seems to reveal a remarkable evolutionary distance between the plastids of the red algae (rhodoplasts), chloroplasts and cyanobacteria.