The capacity of stationary phase cultures of Schizosaccharomyces pombe to survive a heat treatment at 55 degrees C is controlled by a circadian rhythm. In a synchronizing light-dark-cycle this rhythm shows a stable phase relationship to the onset of light. In continuous darkness it persists for several cycles without marked damping. The free-running period of about 27 h at 30 degrees C is only slightly longer at 20 degrees C, hence temperature-compensated. These results indicate that S. pombe is a suitable experimental organism for further research into both heat tolerance and circadian rhythms.