Aims: To determine the potential for Bacillus stearothermophilus cells to form biofilms of significance in dairy manufacture. Methods and Results: The ability of isolates of B. stearothermophilus from dairy manufacturing plants to attach to stainless steel surfaces was demonstrated by exposing stainless steel samples to suspensions of spores or vegetative cells and determining the numbers attaching using impedance microbiology. Spores attached more readily than vegetative cells. The attachment of cells to stainless steel was increased 10-100-fold by the presence of milk fouling the stainless steel. The growth of B. stearothermophilus as a biofilm on stainless steel surfaces was determined using a continuously flowing experimental reactor. Vegetative cells were released in greater numbers than spores from biofilms of most strains studied. Biofilms of one strain (B11) were studied in detail. Biofilms of >10(6) cells cm(-2) formed in the reactor and released approximately 10(6) cells ml(-1) into milk passing over the biofilm. A doubling time of 25 min was calculated for this organism grown as a biofilm. Conclusions: The formation of biofilms of thermophilic Bacillus species within the plant appears to be a likely cause of contamination of manufactured dairy products. Methods to control the formation of biofilms in dairy manufacturing plants are required to reduce the contamination of dairy products with thermophilic bacilli. Significance and Impact of the Study: Biofilms of B. stearothermophilus growing in dairy manufacturing plants can explain the contamination of dairy products with these bacteria.