In a chlorophyll(Chl)-deficient ch5 mutant of Arabidopsis thaliana the thylakoid membrane of which is more loosely arranged than that of the wild type we characterized the xanthophyll cycle (VAZ cycle) components (violaxanthin - V, antheraxanthin - A and zeaxanthin - Z) in comparison with the wild type grown under three low irradiances. As the irradiance increased from 30 to 250 pmol m'^ s'i, the total amount of components of the VAZ cycle decreased in both biotypes, but those of the ch5 mutant decreased more slowly than did those of the wild type. In both biotypes, the relative amoímts of V and A decreased as the irradiance increased, and that of Z increased, but the increase and decrease in the ch5 mutant was slower than in the wild type. The epoxidation index showed that the VAZ cycle operated at a similar efficiency in both biotypes (between 30 and 250 pmol m‘2 s'*), but that the ch5 mutant worked at a capacity 3.5-13.5 % lower than did the wild type, depending on the irradiance. Thus in Chl-deficient mutants, the VAZ cycle operates at a lower capacity than in the wild type, and the mutants are more sensitive to the changes in irradiance.