Alcoholic extract of the marine algae Chlorella vulgaris was examined for its free radical scavenging effect with reference to naphthalene-induced lipid peroxidation in serum, liver, and kidney of rats. Initially, upon naphthalene intoxication (435 mg/kg body weight, intraperitoneally), the lipid peroxidation activity increased signiﬁcantly (P < 0.001), and in contrast, the enzymic antioxidants (superoxide dismutase, catalase, glutathione peroxidase) and non-enzymic antioxidants (glutathione, ascorbic acid, and a-tocopherol) levels decreased remarkably. When the naphthalene stressed rats were treated with Chlorella vulgaris extract (70 mg/kg body weight, orally), the lipid peroxidation activity reduced signiﬁcantly (P < 0.001) and the activities of both the enzymic and non-enzymic antioxidants increased reaching near control values. The minimum concentration (70 mg/l) of the extract that exhibited maximum (85%) free radical scavenging activity was chosen for the experimental study. The present results suggest that Chlorella vulgaris extract exerts its chemo-preventive effect by modulating the antioxidants status and lipid peroxidation during naphthalene intoxication.