Chemostat cultures of Bacillus stearothermophilus cells using starch as carbon source achieved maximum cell productivity at a dilution rate of 1.0 h(-1) with 0.75 g dry weight l(-1) h(-1). Amylase specific activity ranged from 112 nkat mg(-1) to nearly nil values, decreasing when changing dilution rate from 0.1 to 1.5 h(-)1 and leading to a starch concentration increase from 3 to 9 g l(-1). The specific substrate consumption rate increased tenfold (1.04-11.2 g g(-1) h(-1)) with the dilution rate within the range 0.1- 1.5 h(-1). Starch pulses at low dilution rate (0.2 h(-1)) produced an increase in specific growth rate (0.26 h(-1)). Conversely, at high dilution rate (1.0 h(-1)), there was a marked decrease in the specific growth rate (0.3 h(-1)). At both dilution rates (0.2 and 1.0 h(-1)), the steady-state cultures showed a decrease in cell yield and an increase in specific substrate consumption rate. Nevertheless, slow-growing cells (0.2 h(-1)) showed a double specific substrate consumption rate with half of the cell yield as compared with fast-growing cells (1.0 h(-1)). Both fast- and slow-growing cells showed a decrease in dry weight during temperature shifts. Reducing sugar concentration remained constant whereas a slight increase in starch concentration and a decrease in amylase specific activity was observed. The enzyme recovered the initial values, once the temperature of 60 degreesC was restored. However, slow-growing cells (0.2 h(-1)) showed higher levels of amylase specific activity and, as a result, lower starch concentration than fast-growing cells (1.0 h(-1)) (C) 2002 Elsevier Science Ltd. All rights reserved.