Technical grade material and wettable powder formulations of the insect growth regulator diflubenzuron and 3 of its degradation products were tested for toxicity to 3 species of aquatic invertebrates and 4 fishes: daphnids (Daphnia magna), scuds (Gammarus pseudolimnaeus), midges (Chironomus plumosus), rainbow trout (Salmo gairdneri), fathead minnows (Pimephales promelas), channel catfish, (Ictalurus punctatus), and bluegills (Lepomis macrochirus). The acute toxicities of the wettable power formulation of diflubenzuron ranged form a 48-hr EC50 (estimated concentration immobilizing 50% of test organisms) of 0.015 mg/liter for daphnids to a 96-hr LC50 (estimated concentration producing 50% mortality) of 660 mg/liter for bluegills. The 96-hr LC50 of the technical grade material exceeded 100 mg/liter for all 4 fishes. The most toxic degradation product, 4-chloroaniline, had a 96-hr LC50 of 2.4 mg/liter to bluegills and a 48-hr EC50 of 43 mg/liter to early fourth-instar midge larvae. The 48-hr EC50's (midge larvae) and 96-hr LC50 for 3 of 4 species of fish for 4-chlorophenyl urea and 2,6 difluorobenzoic acid were greater than 100 mg/liter.