The design of a temperature‐controlled observation chamber is described, that allows light‐microscopic studies of the motile behavior of thermophilic bacteria at temperatures up to 100 °C. The chamber is built for use on an inverted light microscope. Heating and cooling is performed by Peltier elements, with a maximum rate of temperature change of 2 °C/s. The temperature is controlled via a personal computer with an accuracy of ±0.25 °C. ©1994 American Institute of Physics.