Aims: To determine the potential for Bacillus stearothermophilus cells to form bio®lms of signi®cance 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 bio®lm on stainless steel surfaces was determined using a continuously¯owing experimental reactor. Vegetative cells were released in greater numbers than spores from bio®lms of most strains studied. Bio®lms of one strain (B11) were studied in detail. Bio®lms of > 10 6 cells cm À2 formed in the reactor and released approximately 10 6 cells ml À1 into milk passing over the bio®lm. A doubling time of 25 min was calculated for this organism grown as a bio®lm. Conclusions: The formation of bio®lms 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 bio®lms in dairy manufacturing plants are required to reduce the contamination of dairy products with thermophilic bacilli. Signi®cance and Impact of the Study: Bio®lms of B. stearothermophilus growing in dairy manufacturing plants can explain the contamination of dairy products with these bacteria.