Aging and limited life span are fundamental biological phenomena observed in a variety of species . Approximately 55 genes have been identified that can extend longevity when altered in Caenorhabditis elegans [2-5]. These genes include an insulin-like receptor (daf-2) and a phosphatidylinositol 3-OH kinase (age-1) regulating a forkhead transcription factor (daf-16) [6, 7], as well as genes mediating metabolic throughput , sensory perception , and reproduction . Moreover, these mutant alleles both extend life span and increase resistance to ultraviolet (UV) radiation , heat , and oxidative stress [13-15], though the stress resistance of clk-1 is controversial. With the exception of old-1 and perhaps some other genes [16-19], all of the life-extension alleles are hypomorphic or nullomorphic. Here, we show that the OLD-1 transmembrane tyrosine kinase (formerly TKR-1; [16, 20]) is expressed in a variety of tissues, is stress inducible, and is a positive regulator of longevity and stress resistance. The transcription of old-1 is upregulated in long-lived age-1 and daf-2 mutants and is upregulated in response to heat, UV light, and starvation. Both RT-PCR and analysis of an OLD-1::GFP tag suggest that old-1 expression is dependent on daf-16. Importantly, old-1 is required for the life extension of age-1 and daf-2 mutants. This study reveals a new system for specifying longevity and stress resistance and suggests possible mechanisms for mediating life extension by dietary restriction and hormesis.