Light, chemicals, and mutations that affect the carotene content of the fungus Phycomyces blakesleeanus had practically no effect on the ergosterol content. Lovastatin, a specific inhibitor of hydroxymethylglutaryl coenzyme A reductase, blocked growth at 1 microM; sodium DL-mevalonate (10 mM) fully reversed this inhibition. In the presence of [14C]mevalonate, a carS mutant accumulated 16 times more beta-carotene than the wild-type with a specific radioactivity five times lower. The specific radioactivity of ergosterol was different from that of beta-carotene, even when calculated in terms of the constituent isoprene units, and unaffected by the carS mutation. The carotene and sterol pathways of Phycomyces are independently regulated and physically separated in different subcellular compartments.