The production of bacterial enzymes in batch fermentations is compared with results obtained in continuous culture. When studying the production of alpha-amylase in Bacillus subtilis it was found that instability of the enzyme synthesis was due to nonhomogeneity of the population rather than to "the culture's history" (i.e. succession of several physiological states necessary for the enzyme production). The plasmid contained in the production clone was found to be the factor responsible for the alpha-amylase production. Predominance of the production clone or of the nonproduction one depends on the cultivation conditions used. As compared with batch cultivation the continuous production yields higher enzyme concentrations under optimal conditions and the fermentor productivity may be four to five times higher.