Eight strains of C. elegans, including seven recombinant inbred (RI) strains with mean life spans ranging from 10.9 to 28.8 days, were reared under two conditions (95% oxygen or methyl viologen) known to elevate the concentration of free radicals. Both agents inhibited development, as ascertained by measuring the lengths of animals at regular intervals. The degree of inhibition correlated inversely with mean life span with both agents; that is, development of short-lived strains was inhibited more profoundly than was development of long-lived strains. Thus, at least some of the polygenes which prominently influence aging are the same as those which control resistance to free radicals. These genes likely influence aging and response to oxidative stress in many ways, both direct and indirect.