Chlorophyll -a content of autotrophic cells of Chlorella vulgaris was double that estimated in heterotrophic cells, while chlorophyll -b content of autotrophic cells was nearly half the value recorded for heterotrophic cells. Carotenoids content of heterotrophic cultures decreased by 30.82% compared to that value of autotrophic cells. There was a slight decrease in the protein content of C. vulgaris under heterotrophic conditions. When the composition of total free amino acids and proline of C. vulgaris grown under autotrophic conditions is compared to that grown heterotrophically, it was observed that a significant increase in total free amino acids and proline in heterotrophic cultures. The percentage of most fatty acids of heterotrophic cells was relatively higher than autotrophic ones. There was no qualitative difference between autotrophic and heterotrophic cultures, except for the fatty acid 16:02 which was absent under autotrophic conditions.