An insulin-like signaling pathway regulates development and lifespan in Caenorhabditis elegans. Genetic screens that identified many components of the C. elegans insulin pathway did not identify homologs of insulin receptor substrates or the phosphoinositide 3-kinase (PI3K) adaptor/regulatory subunit, which are both required for signaling by mammalian insulin/insulin-like growth factor I pathways. The C. elegans genome contains one homolog of each protein. The C. elegans versions of insulin receptor substrate (IST-1) and PI3K p50/p55 (AAP-1) share moderate sequence similarity with their vertebrate and Drosophila counterparts. Genetic experiments show that ist-1 and aap-1 potentiate C. elegans insulin-like signaling, although they are not required for signaling in the pathway under most conditions. Worms lacking AAP-1 activity because of the mutation aap-1(m889) constitutively arrest development at the dauer larval stage when raised at high temperatures. aap-1 mutants also live longer than wild-type animals, a phenotype observed in other C. elegans mutants with defects in DAF-2 signaling. Interestingly, IST-1 appears to be required for signaling through a pathway that may act in parallel to AGE-1/PI3K.