Studies on growth and development of Campoletis sonorensis (Cameron) in various instar Heliothis virescens (F.) larvae were conducted. The behavior and morphological changes of the parasitized host was also evaluated. The results indicate that the host’s age at the time of parasitism apparently influenced the development and subsequent fitness of the parasitoid. When initiating development in 3rd or 4th instar hosts, the parasitoid developed significantly slower than when initiating in 1st instar hosts. Furthermore, when developing in 3rd or 4th instar hosts, the parasitoid developed significantly faster during embryonic and 1st instar stages, and significantly slower during the 2nd, 3rd, and 4th instar stages than those developing in 1st instar hosts. In younger hosts, parasitoids also produced a significantly reduced body length, weight, longevity, and egg production estimate. Parasitism of H. virescens larvae by C. sonorensis changed host’s morphology and behavior allowing easy identification of parasitized individuals and estimation of the age of the developing parasitoid within its host.