S. cerevisiae contains a family of genes related to the major heat shock induced gene of Drosophila (Hsp70). Two members of this family, YG100 and YG102, are 97% identical to each other in their protein coding regions. RNA transcribed from YG100 increases markedly after a heat shock, while transcripts of YG102 increase minimally. Mutants of the two genes were constructed in vitro and substituted in the yeast genome in place of the wild-type alleles. No phenotypic effect of single mutations of either gene was detected. However, cells containing both the YG100 and YG102 mutations grew slowly at 30 degrees C and could not form colonies at 37 degrees C. The temperature sensitive phenotype can be overcome by inserting either an intact YG100 or YG102 gene into the genome. Pretreatment at 37 degrees C before shift to a normally lethal temperature of 51 degrees C protected the double mutant as well as the wild type, indicating that YG100 and YG102 gene products are not needed for resistance to high temperatures for short periods.