The contamination of milk by thermophilic bacteria is highly undesirable. However, the precise bacterial behaviour in milk is not been well quantified. In this study, Bacillus stearothermophihis was used as a representative thermophilic bacterium and the main milk components were used as substrates to study the growth behaviour of this species. The growth of B. stearotherinophilus has been found to promote acidic conditions in both milk and lactose solution. It has been found that milk proteins sustain B. stearothermophilus in stationary phase for longer than other milk components during batch growth. Milk protein solution which contained B. stearothermophilus formed thick, dense porous lumps, more rapidly than that without bacteria. The presence of biological material on heat exchanger surfaces may promote other fouling mechanisms. Therefore, implications of the present study to the influence of the bacteria on milk fouling are also discussed. (c) 2005 Published by Elsevier Ltd.