Single-gene mutations in Caenorhabditis elegans increase thermotolerance (Itt) and extend lifespan. The age-1 gene mutation leads to a 65% increase in lifespan and increased Itt. Long-lived spe-26 and daf-2 mutants also exhibit Itt. Environmental induction of thermotolerance at sublethal temperatures increases both Itt and lifespan, suggesting a causal relationship between these traits. These findings indicate that stress resistance limits lifespan in C. elegans and possibly in other metazoa. Mutations in genes such as age-1, daf-2, daf-4, daf-7, and spe-26 increase Itt and lifespan. The Itt phenotype is genetically linked to increased lifespan, suggesting a common mutational event. Environmental induction of thermotolerance also extends lifespan, supporting a causal relationship between stress resistance and longevity. These results suggest that stress response mechanisms are crucial for lifespan extension in C. elegans and may be conserved in other species. The study highlights the importance of stress response pathways in aging and longevity.Single-gene mutations in Caenorhabditis elegans increase thermotolerance (Itt) and extend lifespan. The age-1 gene mutation leads to a 65% increase in lifespan and increased Itt. Long-lived spe-26 and daf-2 mutants also exhibit Itt. Environmental induction of thermotolerance at sublethal temperatures increases both Itt and lifespan, suggesting a causal relationship between these traits. These findings indicate that stress resistance limits lifespan in C. elegans and possibly in other metazoa. Mutations in genes such as age-1, daf-2, daf-4, daf-7, and spe-26 increase Itt and lifespan. The Itt phenotype is genetically linked to increased lifespan, suggesting a common mutational event. Environmental induction of thermotolerance also extends lifespan, supporting a causal relationship between stress resistance and longevity. These results suggest that stress response mechanisms are crucial for lifespan extension in C. elegans and may be conserved in other species. The study highlights the importance of stress response pathways in aging and longevity.