Carotene synthesis in the Mucoral fungus, Phycomyces blakesleeanus, is regulated by a complex genetic mechanism and activated by four groups of environmental factors with independent mechanisms of action. Blue light and sexual stimulation increased in parallel the content of carotene and the content of mRNAs from the genes, carRA and carB, dedicated to the synthesis of β-carotene from geranylgeranyl pyrophosphate. The effects of these agents were approximately additive. Retinol and dimethyl phthalate, which represent the remaining groups of activators, greatly increased the carotene content, but did not modify the levels of carRA and carB transcripts. Mutants in genes carRA, carB, carC, carD, carF, carI, and carS differed in their carotene content, from nil to much larger than that of the wild type, but had the same carRA and carB transcript levels as the wild type. The only exception was a carRA early-stop mutant, which had very small amounts of the carRA transcript. The genetic and environmental factors that modify carotene biosynthesis had little or no effect on the mRNA levels of genes, hmgS and hmgR, responsible for the enzymes that initiate the biosynthesis of all terpenoids. A general model for the regulation of carotenogenesis in Phycomyces was derived from the results.