none The preimaginal development of the parasitoid Cotesia marginiventris (Cresson) was observed in soybean looper, Pseudoplusia includens Walker, on different genotypes of soybean, Glycine max (L.) Merrill. Larvae were reared from eclosion on resistant soybean breeding lines, ED73-371 and D75-10230, both having PI 229358 as the resistance source, or on susceptible cultivars, 'Bragg' and 'Braxton'. Parasitized larvae consumed less foliage than nonparasitized larvae. Parasitized larvae, regardless of soybean genotype, also weighed less than nonparasitized larvae that fed on susceptible varieties. The developmental time of C. marginventris intrahost was not affected by the hosts' feeding on resistant germplasm. In field cages which contained only one of the soybean genotypes (no choice), parasitization of soybean looper among soybean genotypes was significantly greater in loopers collected from resistant genotypes for one generation during 1986. The resistant genotypes also had greater percentages of undeveloped parasitoids.