The influence of ammonium, phosphate and citrate on astaxanthin production by the yeast Phaffia rhodozyma was investigated. The astaxanthin content in cells and the final astaxanthin concentration increased upon reduction of ammonium from 61 mM to 12.9 mM (from 140 mug/g to 230 mug/g and 1.2 mug/ml to 2.3 mug/ml, respectively). Similarly, both the astaxanthin content and astaxanthin concentration increased by reducing phosphate from 4.8 mM to 0.65 mM (160 mug/g to 215 mug/g and 1.7 mug/ml to 2.4 mug/ml, respectively). Low concentrations of ammonium or phosphate also increased the fatty acid content in cells. By analogy with lipid synthesis in other oleaginous yeasts, an examination of the data for varying nitrogen and phosphate levels suggested that citrate could be the source of carbon for fatty acids and carotenoid synthesis. Supporting this possibility was the fact that supplementation of citrate in the medium at levels of 28 mM or higher notably increased the final pigment concentration and pigment content in cells. Increased carotenoid synthesis at low ammonium or phosphate levels, and stimulation by citrate were both paralleled by decreased protein synthesis. This suggested that restriction of protein synthesis could play an important role in carotenoid synthesis by P. rhodozyma.