Continuous cultivation of Bacillus amyloliquefaciens B155 using a defined synthetic medium, which supports good batch cell growth and alpha-amylase production, was limited by lactose, nitrogen, phosphorus and oxygen. In each case, the organism changed to a non-alpha-amylase-producing variant with morphological characteristics different from the original culture. Reinitiation of alpha-amylase production could not be achieved by plating the variant culture on solid complex medium. alpha-Amylase-producing and non-alpha-amylase-producing colonies, isolated from nitrogen-limited continuous culture were tested for plasmid DNA. The alpha-amylase-producing cells contained plasmid DNA, whereas non-alpha-amylase-producing cells did not. This suggests that plasmid structural instability (resulting in plasmid DNA loss or plasmid copy number reduction by metabolic control) accompanies loss of alpha-amylase production by B. amyloliquefaciens during continuous fermentations.