Data are reported for tests exposing a small, estuarine crustacean, Mysidopsis bahia, to diflubenzuron (Dimilin, TH-6040, (1-(4-chlorophenyl)-3-(2,6-difluorobenzoyl)urea)) in flowing seawater. Tests were conducted in intermittent flows from a diluter or continuous flowing water in which the toxicant was added by an infusion pump. Diflubenzuron was acutely and chronically toxic to M. bahia: the calculated 96-hr LC50 was 2.1 micrograms/l (95% fiducial limits, 1.6 to 2.7 micrograms/l at 24 to 25C and 23 to 29 parts per thousand salinity. In a life-cycle experiment, the 21-day LC50 was 1.24 micrograms/l (95% fiducial limits, 0.84 to 1.8 micrograms/l at 24 to 26C and 23 to 29 parts per thousand salinity). In the life-cycle studies, the reproductive success (number of young produced per female) was found to be a more sensitive criterion of effect than survival of adults. For example, only 13.5 young/female were produced in an estimated concentration of 0.075 micrograms/l whereas 21.4 and 21.0 young/female were produced in controls. As diflubenzuron concentrations increased, there was direct suppression of reproduction.