Wild strains of Rhodotorula glutinis and R. rubra were investigated concerning their carotenoid production, proportion of beta -carotene and cell mass yield. R. glutinis NCIM 3353 produced 2.2 mg carotenoid/l in 72 h; and the amount of beta -carotene was 14% (w,w) of the total carotenoid content (17 mug/g cell dry weight). It was subjected to mutagenesis using UV radiation for strain improvement. Out of 2,051 isolates screened, the yellow coloured mutant 32 produced 120-fold more beta -carotene (2,048 mug/g cell dry weight) than the parent culture in 36 h, which was 82% (w/w) of the total carotenoid content. Mutant 32 was grown on different carbon and nitrogen sources. The best yield of beta -carotene (33 +/-3 mg/l) was obtained when glucose and yeast extract were supplied as carbon and nitrogen sources, respectively. Divalent cation salts further increased the total carotenoid content (66 +/-2 mg/l) with beta -carotene as the major component (55 +/-2%, w/w).