The effect of external CO₂ concentrations on the protein synthesis in Chlamydomonas reinhardtii wild-type is compared with that of a new high CO₂-preferring mutant, designated as pyr-45. Radiolabeled wild-type and pyr-45 cells exhibit up-regulation of two polypeptides (42-45kDa) when adapted from high (5%CO₂ in air) to low CO₂ (0.03%), and wild-type induces three new ones (21, 36 37 kDa), but pyr-45 induces no new polypeptides. Total proteins from pyr-45 mutant cells do not crossreact with antibodies against the three low CO₂-inducible polypeptides of wild-type. The CO₂ requirement for half maximal rates of photosynthesis decreases when pyr-45 cells are switched from high to low CO₂, but not to the extent of wild-type cells. When exogenous carbonic anhydrase (CA) is added to these partially adapted cells,the CO₂ requirement is further reduced, but still not completely. The up-regulation of the 42-45kDa polypeptides under low CO₂ growth conditions suggests these changes play a role in the adaptation of algal cells to limiting CO₂ concentrations in the environment and in the function of the CO₂ concentrating mechanism (CCM) in Chlamydomonas reinhardtii.