Like other carotenoid-producing organisms, Phaffia rhodozyma, a red astaxanthin-producing yeast, is supposed to synthesize carotenoids by the following steps : formation of phytoene from geranylgeranyl pyrophosphate, dehydrogenation of phytoene to lycopene, cyclization of lycopene to beta-carotene and oxidation of the latter to astaxanthin. Mutagenic treatments generated in P. rhodozyma a wide diversity of colour variants ranging from white to dark red. The identification of the corresponding carotenoid compounds revealed the occurrence of beta-carotene-accumulating strains, phytoene-accumulating strains, and strains lacking any carotenoid compound. These classes of strains are likely to result from alterations in, respectively, the oxidation of beta-carotene, phytoene dehydrogenation and the phytoene synthetase step. Except for the cyclization of lycopene to beta-carotene, all the steps of carotenogenesis in P. rhodozyma are represented by the above mutants. Furthermore, astaxanthin-overproducing mutants were also selected; they are likely to be affected in some upstream step, and certainly before beta-carotene, as after an additional mutagenesis they generated oxidaseless strains that, in this case, overproduce beta-carotene. The latter strains appear very promising for biotechnological production of natural beta-carotene.