The effects of repetitive 24-hr weekly exposures to diflubenzuron (Dimilin®) on juvenile fiddler crabs (Uca pugilator) were studied in static seawater systems for 10 weeks. Crabs surviving the 10-week exposure period were maintained in clean seawater until death. Survival, molting, and behavior were monitored daily. The no-effect concentration (NOEC) for molting (time to the first molt), survival (time until death), and behavior (ability to escape from the test container) were 20, 2, and 0.2 μg/L DFB, respectively. The behavioral effect induced by diflubenzuron (DFB) exposures (>2 μg/L) was the most sensitive indicator of DFB activity and potentially may influence the ability of juvenile crabs to avoid predation, construct burrows, or feed adequately to survive in nature.