Exponentially growing cells of the fission yeast, Schizosaccharomyces pombe, contained virtually no trehalose at 27 degrees C but rapidly accumulated large quantities during heat shock at 40 degrees C. Activities of trehalose-6-phosphate synthase and trehalase also increased upon heat shock. Thermotolerance of the cells, measured as survival at 52 degrees C, increased in parallel to trehalose accumulation and decreased in parallel to the trehalose levels when cells were shifted back to 27 degrees C. Trehalose levels, activities of enzymes of trehalose metabolism and thermotolerance strongly increased upon heat shock even in the presence of cycloheximide, indicating that none of these effects requires protein synthesis. The data support the hypothesis that trehalose acts as a thermoprotectant in Schizosaccharomyces pombe.