- The changes in pigment content and composition of the unicellular alga Parietochloris incisa comb. nov (Trebouxiophyceae, Chlorophyta) were studied. This alga is unique in its ability to accumulate high amounts of arachidonic acid in the cell during cultivation under different irradiances and nitrogen availability in the medium. Under low irradiance of 35 µ E/(m 2 s) photosynthetically active radiation the P. incisa cultures possessed slow growth and a relatively low carotenoid-to-chlorophyll ratio. At higher irradiances (200 and 400 µ E/(m 2 s)) on complete medium, the alga displayed higher growth rate and an increase in the carotenoid content, especially that of β -carotene and lutein. Both on nitrogen-free (regardless of illumination intensity) and nitrogen-replete medium (under high light), a considerable increase in the ratio of carotenoid and chlorophyll contents was recorded. Predominant accumulation of xanthophylls took place in thylakoid membranes, whereas β -carotene deposition occurred mainly in the cytoplasmic lipid globules (oil bodies); lower amounts of caro- tenoids were accumulated in the absence of nitrogen. Under high light and nitrogen-deficiency conditions, an increase in violaxanthin de-epoxidation and nonphotochemical quenching was recorded together with a decline in variable chlorophyll fluorescence ( F v / F m ) level. A possible photoprotective role of carotenoids in adaptation of P. incisa to high light under nitrogen starvation conditions is discussed.