The present study deals with the microbiological transformation of L-tyrosine to 3,4-dihydroxyphenyl L-alanine by a mutant strain of Aspergillus oryzae UV-7. Sixteen different mutant strains of Aspergillus oryzae (GCB-6) were isolated through UV-irradiation. These mutant strains were screened for the production of mold mycelia by submerged fermentation in 250-ml Erlenmeyer flasks. Of all the mutant strains examined, UV-7 gave maximum production of L-dopa (1.28 mg/ml). The reaction was carried out using mold mycelium as a source of enzyme tyrosinase in shake flasks. The maximum production of L-dopa was obtained when glucose (25 mg/ml) was used as the carbon source and NH4Cl (3 mg/ml) was used as the nitrogen source. The optimum pH for mycelium development was 5.0; L-dopa production was maximum at pH 3.5 of the reaction mixture. The reaction by mold mycelium (75 mg/ml) was carried out under acidic conditions. Optimum temp, time, and L-tyrosine concentration were 55degreesC, 60 min, and 3.0 mg/ml, respectively.