A quantitative and temporal correlation was found between the production of avermectins and lipids in Streptomyces avermitilis grown in defined media. The lipid fractions contained mostly triglycerides and phospholipids. A significant increase in the content of avermectins and triglycerides was observed during the stationary phase. The syntheses of both of these compounds stopped simultaneously with the depletion of glucose from medium. The results indicate that systems synthesizing avermectins and fatty acids could be competing for common precursors. © 1990 Federation of European Microbiological Societies.