Infective juveniles (Us) of entomopathogenic nematodes (EPNs) are obligate parasites of insect larvae. Inside the host they develop into sexually mature adult stages and complete their life cycle. Two or 3 adult nematode generations can occur in the insect host. The increase in nematode population density in the insect cadaver, together with limiting nutrient conditions, result in the formation of IJs. These IJs emerge into the soil to search for a new host. It typically takes 7-8 days for all Us to emerge from a parasitized insect. We have investigated the effect of the day of emergence of Us from insect cadavers on the environmental tolerance and behavior of the EPN Heterorhabditis megidis strain UK211. The Us that emerge early display good initial host-finding ability and increased temperature tolerance but disperse poorly and have poor tolerance to desiccation. Conversely, the IJs that emerge later display poor initial host-finding ability and poor temperature tolerance but they disperse well and possess increased desiccation tolerance. These phenotypic differences are likely to facilitate early-emerging Us in locating and infecting hosts in the vicinity of the cadaver, whereas IJs that emerge late are adapted to disperse away from their natal cadaver. We hypothesize that adaptive phenotypic plasticity rather than allelic variability may provide the genetic basis for the different physiological and behavioral phenotypes of the early- and late-emerging IJs.