Mechanisms for the undesired persistence of Bacillus species in paper machine slimes were investigated. Biofilm formation was measured for industrial Bacillus isolates under paper machine wet-end-simulating conditions (white water, pH 7, agitated at 45 degrees C for 1-2 days). None of the 40 tested strains of seven Bacillus species formed biofilm on polished stainless steel or on polystyrene surfaces as a monoculture. Under the same conditions, Deinococcus geothermalis E50051 covered all test surfaces as a patchy thick biofilm. The paper machine bacilli, however, formed mixed biofilms with D. geothermalis E50051 as revealed by confocal microscopy. Biofilm interactions between the bacilli and the deinococci varied from synergism to antagonism. Synergism in biofilm formation of D. geothermalis E50051 was strongest with Bacillus coagulans D50192, and with the type strains of B. coagulans, B. amyloliquefaciens or B. pumilus. Two B. licheniformis, one B. amyloliquefaciens, one B. pumilus and four B. cereus strains antagonized biofilm production by D. geothermalis. B. licheniformis D50141 and the type strain of B. licheniformis were the strongest antagonists. These bacteria inhibited deinococcal growth by emitting heat-stable, methanol-soluble metabolite(s). We conclude that the persistence of Bacillus species in paper machine slimes relates to their ability to conquer biofilms formed by primary colonizers, such as D. geothermalis.