The burrowing activity of Uca pugilator in sand in laboratory trays was not altered when the sand was contaminated with 1.0 rng 1-' diflubenzuron (DFB), indicating lack of avoidance of DFBcontaminated sand. However, crabs that had been exposed to DFB concentrations as low as 0.5 pg 1-' for 1 wk or more exhibited a reduction in burrowing activity, as measured by the number of burrows dug at 15 and 60 min after release and the number of crabs within burrows at those times. The reduction in burrowing activity was not concentration-dependent between 0.5 and 50 pg I-' DFB, and was unchanged between 1 and 3 wk of exposure to the pesticide. Since fiddler crabs are dependent on burrows for many aspects of their lives, the reduction of this behavior may have serious consequences for the species.