The purpose of this work was to test whether induction of massive beta-carotene synthesis in the alga Dunaliella bardawil is triggered by oxygen radicals. The following results were obtained: (i) The induction of beta-carotene synthesis is preceded by a lag period of about 4 h during which the cells swell and photosynthesis is partially inhibited. (ii) Addition of promoters of oxygen radicals or of azide (an inhibitor of catalase and superoxide dismutase) during the induction period, under conditions which are suboptimal for massive beta-carotene accumulation, greatly enhances beta-carotene synthesis, photodegradation of chlorophyll and inhibition of photosynthesis. (iii) High irradiance, which induces massive beta-carotene accumulation, also induces a high catalase activity. It is suggested that photosynthetically produced oxygen radicals are involved in triggering massive carotene accumulation in D. bardawil.