The purpose of this research was to determine the effect of the size of the host,Anthonomus grandis grandisBoheman, on adult size, sex ratio, longevity, and fecundity of the ectoparasitoidBracon mellitorSay. Adult size ofB. mellitorwas directly related to host size; larger parasitoids developed on larger host larvae. Host size also affected sex ratio so that predominately male wasps were produced on small hosts. The male bias on smaller hosts was caused by differential oviposition of male and female eggs by adult females. Female longevity and fecundity were directly related to adult size; larger females lived longer and subsequently produced greater numbers of progeny than smaller females.