In the multicellular red alga Antithamnion spec. both rubisco genes (rbcL and rbcS) are encoded on the plastid DNA (ptDNA). Both genes are separated by a short A/T-rich spacer of 100 bp and are cotranscribed into an mRNA of approximately 2.7 kb. These findings are in extensive agreement with those obtained from two unicellular red algae (Porphyridium aerugineum and Cyanidium caldarium). The large subunit (LSU) of rubisco shows an amino acid homology of 82-87% with the LSUs from the two unicellular red algae and only about 55% to LSUs from green algae, higher plants and two cyanobacteria. The small subunit (SSU) of rubisco is more similar to those from the unicellular red algae and two algae which are members of the Chromophyta (about 60% homology) than to cyanobacterial and higher plant proteins (27-36% homology). These data indicate that rhodoplasts originated independently from the chloroplast line. The plastids of chromophytes and rhodophytes appear to be closely related.