The filamentous fungus Blakeslea trispora, an industrial carotene source, contains beta-carotene and precursors of its synthesis - phytoene, phytofluene, lycopene, and gamma-carotene. Strain improvement through mutagenesis is difficult because all life stages are multinucleate. Mutants have been obtained following exposure of wild-type spores to N-methyl-N’-nitro-N-nitroso-guanidine. Changes in the colour of the mycelia reflect variations in the accumulation of various precursors and the final product. Quantitative analysis of the mutants leads to the conclusion that the biosynthetic pathway is similar to that of the related fungus Phycomyces blakesleeanus, but the regulation is completely different. In particular, interruption of the pathway does not lead to overacummulation of precursors.