THEORIES Of life-history evolution propose that trade-offs occur between fitness components, including longevity and maximal reproduction(1-3). In Drosophila, female lifespan is shortened by increased egg production(4), receipt of male accessory fluid(5) and courting(6), Male lifespan is also reduced by courting and/or mating(7). Here we show that in the nematode Caenorhabditis elegans, mating with males reduces the lifespan of hermaphrodites by a mechanism independent of egg production or receipt of sperm, Conversely, males appear unaffected by mating, Thus, in C. elegans there is no apparent trade-off between longevity and increased egg or sperm production, but there is a substantial cost to hermaphrodites associated with copulation.