AbstractLipid peroxidation and decrease in chlorophyll‐aand chlorophyll‐bcontent inScenedesmuscells was followed in the course of time. Addition of diquat to the algae in the light causes lipid peroxidation and a decrease in chlorophyll content. This decrease is mainly due to chlorophyll‐a, the concentration of chlorophyll‐bremains more or less constant during the experiment. In the presence of N′‐(3,4‐dichlorophenyl)‐NN‐dimethylurea (DCMU), of cysteine, or during nitrogen‐flushing of the algal suspension, the lipid peroxidation caused by diquat is strongly suppressed. The decrease in chlorophyll‐acontent caused by diquat is somewhat smaller in the presence of DCMU or during nitrogen‐flushing than with diquat alone, but is not influenced by cysteine. The chemical antioxidant butylated hydroxytoluene does not affect lipid peroxidation and chlorophyll destr