A partially constitutive mutant strain for penicillin amidase production was derived from the parent strain Bacillus megaterium ATCC 14945 by treatment with UV light. The mutant (B. megaterium KFCC 10029) showed two phenotypical changes in the mode of penicillim amidase production and in the size of cell chains. While the parent strain produced penicillin amidase only in the presence of an inducer, phenylacetic acid, the mutant strain could produce the enzyme without the inducer and the enzyme titer increased three to four times as much as that in the presence of the inducer. The mutant appeared as isolated single cells or short chains on the nutrient agar medium, whereas the parent strain usually appeared as long chains of cells.The composition of media was optimized including the inducer concentration. After finding the optimal sets of operating conditions with regard to the pH adjustment and the inducer addition time in a submerged culture, we were able to increase the enzyme productivity 7-8 times that without pH control. Although the correlation between two phenotypical changes is not yet clear, the mutant strain can be used as a potent producer of penicillin amidase.