Larvae of Heliothis armigera conferta Walker were sampled on tomatoes and soybeans in areas with and without the recently established parasite, Cotesia kazak (Telenga) (= Apanteles kazak Telenga 1949). Larvae were sorted into 5 size classes to determine the effect of parasitism on the size distribution of larval populations. For average levels of parasitism (42-52%) significantly fewer larvae reached the larger size categories which cause most direct damage of the fruit. Pukekohe sites with C. kazak produced significantly higher proportions of undamaged fruit than Hamilton sites without parasites. In tomatoes, which have low damage tolerance, and increased larval mortality caused by parasitism still left unacceptably high levels of damage. C. kazak can decrease the damage caused by H. armigera and provides useful control in crops in which some damage is tolerated. none