The industrial production of beta-carotene with the zygomycete Blakeslea trispora involves the joint cultivation of mycelia of opposite sex in the presence of beta-ionone and other chemical activators. We have obtained improved strains by mutation and heterokaryosis. We chose wild strains on the basis of their growth and carotene content in single and mated cultures. Following exposure of their spores to N-methyl-N'-nitro-N-nitrosoguanidine, we obtained high-carotene mutants, which were more productive than their parents but similar to them in having beta-carotene as the main product. Further increases in carotene content were obtained after a new round of mutagenesis in one of the mutants. The production was shifted to lycopene in cultures incubated in the presence of nicotine and in lycopene-rich mutants derived from the wild strains. The highest production levels were achieved in intersexual heterokaryons, which contained mutant nuclei of opposite sex. These contained up to 39 mg of beta-carotene or 15 mg of lycopene per g (dry mass) under standard laboratory conditions in which the original wild strains contained about 0.3 mg of beta-carotene per g (dry mass). Beta-ionone did not increase the carotene content of these strains. Not all wild strains lent themselves to these improvements, either because they produced few mutants or because they did not increase their carotene production in mated cultures.