Field populations ofHeliothis spp. were sampled for levels of naturally occurring larval parasitism on six tomato lines varying in levels of 2-tridecanone-mediated resistance toManduca sexta (L.) andLeptinotarsa decemlineata (Say). Second and third instars were parasitized byCampoletis sonorensis (Cameron) (Hymenoptera: Ichneumonidae) andCotesia (=Apantales)marginiventris (Cresson) (Hymenoptera: Braconidae) in 1984 through 1986 and byMicropletis croceipes (Cresson) (Hymenoptera: Braconidae) in 1986. Differences in parasitism by individual and multiple species among host plants were not demonstrated. However, levels of parasitism were low and variable among replicates. Total larval parasitism averaged across all plant lines was less than 6% in 1984 and 1986 and approximately 11% in 1985. In laboratory cage studies,C. sonorensis parasitized fewerH. zea larvae on tomato foliage with high levels of 2-tridecanone than on foliage with low levels. RearingH. zea on diet containing 2-tridecanone and 2-undecanone did not alter incidence of parasitism byC, sonorensis; nor did rearing parasitizedH. zea larvae on chemically treated host diets precondition the parasitoid to higher or lower mortality when transferred to foliage as a substrate for cocoon spinning, regardless of the foliage genotype. However, parasitoid survival during cocoon spinning on foliage varied significantly among plant lines in a manner corresponding to the level of 2-tridecanone-mediated resistance of the foliage. Parasitoid mortality was greatest on highly resistant foliage and lowest on susceptible foliage.