A laboratory experiment showed that preferences ofMicroplitis croceipes(Cresson) for certain larval instars ofHeliothis virescens(F.) may arise in part from variation in probability of parasitization once a parasitoid encounters a host. Host acceptance was higher and handling time shorter for preferred host instars. In support of the hypothesis that parasitoids choose host stages that maximize fitness, development rate was faster and adult head width of wasps was larger for wasps from hosts parasitized in preferred instars. However, the number of oocytes was smallest in wasps from hosts parasitized in the most preferred instar, suggesting a trade-off between development rate and fecundity. Furthermore, egg/larval mortality, pupal mortality, and adult mortality increased monotonically with host instar parasitized. Although the fitness components measured did not relate simply to instar preferences, reasons were found for avoidance of first instar and fifth instar hosts, the least preferred instars.