SUMMARY: Escherichia coli can be rendered competent for DNA uptake by exposure to a heat shock in the presence of divalent cations. We have studied the influence of variations of the incubation temperature in the competence regimen on the efficiency of competence induction, i.e. the efficiency of uptake of DNA into a DNAase resistant form. For cells grown at 37 °C DNA uptake occurs (1) during a heat shock from 0 °C to temperatures between 15 °C and 42 °C (optimal, 30 °C) and (2) after a heat shock from 0 °C to temperatures between 20 °C and 42 °C (optimal, 32 °C) and a subsequent cold shock to 0 °C. Under the latter conditions DNA uptake occurs during incubation of the transformation mixture at 0 °C after the heat shock. In both cases the efficiency of DNA uptake increases as the incubation temperature during the heat shock increases from about 18 °C to about 32 °C. When recipient cells are grown at 22 °C instead of at 37 °C, the temperature range at which competence induction occurs is shifted by 5 °C to lower temperatures. These results indicate that phase transitions of membrane lipids may play a critical role in induction of competence. When recipient cells are shocked from high temperature to low temperature, leakage of the periplasmic β-lactamase occurs; the degree of leakage and the efficiency of competence induction are affected similarly by the temperature range of the shock. This observation indicates that phase transition of membrane lipids causes damage to the outer membrane, and that this damage may be essential for induction of competence.