Two fish and seven invertebrate freshwater species were exposed to diflubenzuron (Dimilin) in acute and chronic laboratory tests. No effects on newly hatched and juvenile fathead minnows or juvenile guppies were seen at or below 36 micrograms/L, the highest concentration tested. An early-life-stage test (30-d) with fathead minnows showed no effect at or below 36 micrograms/L. No effects on survival, growth or reproduction were observed with two snail species, Juga plicifera and Physa spp., at or below 36 micrograms/L. Adult emergence of the caddis fly Clistoronia magnifica was inhibited at 0.1 micrograms/L. Molting and survival of the midge Tanytarsus dissimilis were affected at 4.9 micrograms/L, and adult emergence did not occur at 1.6 micrograms/L.