Ovigerous grass shrimp (Palaemonetes pugio) carrying 0.5-, 1-, 3-, 6-, and 8-day-old embryos (i.e., stages 1, 2, 3, 4 and 5, respectively) were exposed continuously for 4 days to a single dose of difiubenzuron (DFB) at sublethal concentrations (0.3 to 5.0 νg/L) in a static system. After the 4-day exposure, the shrimp were transferred to DFB-free seawater for the rest of the embryonic development. When the eggs hatched, the following toxicity-endpoints were measured: Hatchability (% hatch), larval viability (% of larvae surviving to the postlarval stage), duration of larval development from hatching to the postlarval stage, and severity of morphological abnormality in the larvae after hatching. There was no correlation between the age of the embryos at exposure and either hatchability or duration of larval development. Also, severity of abnormality did not vary with age of embryos except at an exposure concentration of 2.5 νg/L. However, within each age group of the embryos, the severity of larval abnormality was and duration of larval development was concentration dependent. Larval viability was significantly (P < 0.05) affected by the age of the embryos at the time of exposure to DFB. For all the test concentrations, exposure of 0.5- and 1-day-old embryos resulted in larval viability that was similar to the control group (viability > 80%). However, when 6- and 8-day-old embryos were exposed to DFB concentrations > 0.5 νg/L, larval viability was significantly (P < 0.05) less than the controls. These results indicate that older embryos (at more advanced stages of development) of the grass shrimp are more sensitive to sublethal DFB concentrations.