To examine the role of sulfoquinovosyl diacylglycerol (SQDG) in thylakoid membranes, we compared the structural and functional properties of photosystem II (PSII) between a mutant of Chlamydomonas reinhardtii defective in SQDG ( hf-2) and the wild type. The PSII core complex of hf-2, as compared with that of the wild type, showed structural fragility when solubilized with a detergent, dodecyl beta- d-maltoside, suggesting that the physical properties of the PSII complex were altered by the loss of SQDG. On the other hand, exposure of the cells to 41 degrees C for 120 min in the dark decreased the PSII activity to 70% and 50% of the initial levels in the wild type and hf-2, respectively, which implies that the PSII activity, in the absence of SQDG, becomes less stable under heat-stress conditions. PSII inactivated to 60% of the initial level by dark incubation at 41 degrees C was reactivated by following illumination even at 41 degrees C to more than 90% in the wild type, but only to 70% in hf-2. These results suggest that PSII inactivated by heat recovers through some mechanism dependent on light, and that SQDG participates in functioning of the mechanism. The conformational disorder of PSII caused by the defect in SQDG might be correlated with the increased susceptibility of its activity to heat-stress.