We designed a novel procedure for the isolation of mutant strains with significantly increased life spans in the nematode Caenorhabditis elegans. This procedure involves using heat-shock to screen a large number of animals and isolate a few which are more resistant to heat-shock stress. From the heat-shock-resistant animals, three mutant strains, HG25, HG96, and HG246, all exhibiting increased life span, were isolated. One mutant strain (HG246) develops more slowly than the wild-type strain, N2. Two mutant strains, HG96 and HG246, exhibit lower fertility than the wild-type. Each of the three mutant strains has a normal appearance. Their locomotive behavior also appears normal; only HG246 shows slightly slower movement. Their feeding behavior appears normal, and the males of HG25 and HG96 show normal mating behavior. However, the males of HG246, either are defective in their mating ability or their sperm are defective. The results indicate that heat-shock can be used as a means to facilitate the isolation of mutants which have longer life expectancy.