: The insect growth regulator diflubenzuron (DFB) may have substantial effects on non-target organisms, especially crustaceans. By targeting the moulting process, DFB would be expected to elicit the most obvious effects on subadult crustaceans which moult frequently. Previous studies in this laboratory have demonstrated acute and chronic effects of DFB on survival and development of nauplii of the estuarine copepod Eurytemora affinis at water concentrations of less than 1 p.p.b. The present study examines the effects on maturation to adulthood and brood production. Effects on brood production were observed at 0.8 p.p.b. DFB in individuals exposed only during the copepodite stages. Significant effects on production of nauplii were documented only in the early exposure to 0.93 p.p.b. DFB. Inability of post-moult copepodites to completely cast exuviae also was documented in the exposed treatments. At salinities of 2, 10 and 15 p.p.t., survival from naupliar to adult stages was significantly reduced at 0.84 p.p.b. and none survived to adulthood at 1.7 p.p.b. These results suggest that DFB has substantial effects on survival and development of E. affinis at a water concentration of less than 1 p.p.b. DFB concentrations exceeding this concentration have been reported from Chesapeake Bay surface waters although there is no current evidence of potentially toxic concentrations overlapping with the known distribution of E. affinis in the bay.