By UV-irradiation of Eremothecium ashbyii DT1, a highly flavinogenic mutant (UV-18-57) and a nonflavinogenic mutant (UV-85) were obtained. The physio-morphological characteristics of these three strains were studied on glucose medium in submerged fermentation. Glucose utilization and mycelial growth occurred in 0-2 days of fermentation. By the third day, the biomass had declined. Extracellular riboflavin excretion was distinct from the second day, reaching a maximum rate by the fourth day. The hyphae of the highly flavinogenic mutant UV-18-57 were broader than DT1, while the nonflavinogenic UV-85 hyphae were very thin. Riboflavin accumulation was high in UV-18-57 (extracellular riboflavin, 825 μg/ml, and intracellular, 490 μg ml) and caused the mycelia to swell into bulbous forms. Riboflavin accumulation was less in DT1 (108 μg/ml extracellular and 24 μg/ml intracellular) and correspondingly its hyphae were thinner than those of UV-18-57 and swollen bulbous mycelia were not prominent. UV-85 was nonflavinogenic and, accordingly, its morphological characteristics included long thin filaments with no intracellular riboflavin accumulation. A large number of greenish fluorescence spores were seen in UV-18-57, whereas DT1 had less spores and UV-85 was nonsporulating. Sporulation is correlated with riboflavin production. UV-18-57 had better mycelial integrity and lysis started only by the seventh day, whereas DT1 and UV-85 started to lyze earlier by 4-5 days. By the late stage of fermentation (eighth day), DT1 had a few long, thin filaments indicating some secondary growth, whereas UV-85 showed a compact pellet form of mycelia. Most mycelia of UV-18-57 still appeared intact.