By measuring O₂ and CO₂ exchange in mutants of the green alga Chlamydomonas reinhardtii in which genes encoding the reaction center of photosystem I (psaA or psaB) have been deleted, we found that a photosystem II-dependent electron flow using O₂ as the final acceptor can be sustained in the light. However, in contrast with recent reports using other Chlamydomonas mutants (B4 and F8), we show here that CO₂ fixation does not occur in the absence of photosystem I. By deleting the psaA gene in both B4 and F8 strains, we conclude that the ability of these mutants to fix CO₂ in the light is due to the presence of residual amounts of photosystem I.