We examined the function of CAM in reactive oxygen species (ROS) alleviation using a newly isolated CAM-deficient mutant of a facultative halophyte Mesembryanthemum crystallinum L. Salt-stress (0.4 M NaCl) induced nocturnal malate synthesis in the leaves of the wild-type plant, but not in the mutant. The content of hydrogen peroxide (H2O2) increased with elapse of time under salt-stress, but it dropped accompanied by the expression of CAM in the wild-type plants. The CAM-performing wild-type plant grown with 0.4 M NaCl for 12 d showed significantly larger diel fluctuation of malate and significantly lower levels of H2O2 than the mutant, particularly at the end of the light period. The transcript abundance of a gene encoding plastidic Cu/Zn-superoxide dismutase (SOD), a marker of ROS production, was higher in the leaves of mutant plants than in those of wild-type plants. These results indicated that the performance of CAM was accompanied by lower levels of ROS and that CAM may help to alleviate oxidative stress under conditions of environmental stress.