Summary: The permissive temperature for sporulation of Bacillus megaterium 27 (up to 42 °C) was found to be 4–5 °C lower than that for growth. The non-permissive temperature suppressed the initial phases of sporulation characterized by the synthesis of an extracellular proteinase but the cells retained the ability to sporulate for several hours. Neither growth at supraoptimal temperatures nor heat shock applied at the end of the growth phase increased the permissive sporulation temperature. The organism synthesized at least ten heat-shock proteins, the dominant one being HSP 69. These proteins were also found in cells after 3 h of incubation at 43·5 °C but their presence did not ensure the ability to sporulate at this temperature. The rise of temperature provoked an imbalance between synthesis and degradation of cellular proteins, whose role in suppression of sporulation is discussed.