The insect growth regulator (IGR), diflubenzuron (Dimilin 2L), was evaluated for its effect on development of Copidosoma floridanum (Ashmead) and its host, Pseudoplusia includens (Walker), when applied at 0.035 kg ai/ha to soybean, Glycine max (L.), a rate used in preventive programs for control of velvetbean caterpillar, Anticarsia gemmatalis Hübner. Parasitized and nonparasitized soybean looper larvae were fed either diflubenzuron-treated or nontreated foliage. The feeding period of parasitized larvae was 1.7 and 1.8 d longer on diflubenzuron-treated and nontreated foliage, respectively, than that of nonparasitized larvae. Pupation period of C. floridanum and soybean looper was not significantly influenced by diflubenzuron. Although parasitized soybean looper fed both foliage treatments experienced 100% mortality, nonparasitized soybean looper fed nontreated foliage was 23.2% and nonparasitized soybean looper fed diflubenzuron-treated foliage was 12.2%. Parasitized soybean looper fed nontreated foliage consumed 19.2% more leaf area than nonparasitized larvae fed nontreated foliage, whereas parasitized soybean looper fed diflubenzuron-treated foliage consumed 21.2% more leaf area than nonparasitized larvae on the same foliage. There was no significant difference in number of parasitoids emerging from host mummies among treatments; however, there was a positive correlation between leaf area consumed and adult parasitoids per mummy. Diflubenzuron does not appear to produce deleterious effects in C. floridanum or its host, soybean looper, at the current rate used for management of other lepidopteran pests.