Polypeptides from 5% CO2adapted and air-adapted cells ofChlamydomonas segnis, extracted in the presence of the zwitterionic (ampholytic) detergent CHAPS, then lyophilized and washed with a chloroform – methanol mixture, were compared with those extracted in the presence of sodium dodecyl sulphate and Nonidet-P40 and precipitated by cold acetone, using two-dimensional electrophoresis. The electrophoretograms of these detergent-soluble polypeptides indicate that the use of CHAPS instead of sodium dodecyl sulphate – Nonidet-P40 considerably increases the mobility of proteins in the isoelectric focussing gels, improves resolution, and facilitates the detection of polypeptides with low molecular masses on the gradient gels. Comparison of polypeptides extracted with CHAPS from air-adapted and 5% CO2adapted cells revealed the presence of 20 polypeptides unique to the former type of cells. Fourteen of these polypeptides had molecular masses of approximately 20 kDa and isoelectric-point (pI) values ranging from 7.4 to 6.4. They represented the major proteins characteristic of cells capable of active photosynthesis and growth under atmospheric concentrations of CO2.