The dissipation of acaricide chlorobenzilate in selected soil and river water samples was studied. Under incubation conditions at 25 degrees C and 90% field capacity of moisture for 21 days, the residues of chlorobenzilate were 14 and 55% in Lukang silty clay loam and Pincheng clay, respectively. More than 80% of chlorobenzilate remained in the natural water (pH 7.2 and 7.3, respectively) and in the water adjusted to pH 5.0 when water samples collected from Green Lake and Wai-Shuang River near Taipei were treated with chlorobenzilate and then incubated for 34 days. But when the pH was adjusted to 9.0, only 60% was found in Green Lake water and 29% in Wai-Shuang River water. The dissipation of chlorobenzilate in the soil was fitted to first-order kinetics with half-lives of 5 to 36 days under various conditions in the laboratory experiments. No chlorobenzilate was detected in the leachate by a leaching experiment on the soil column for 18 days; most chlorobenzilate remained in the upper 3 cm of the column. According to assessment by the groundwater pollution-potential model (GWP), chlorobenzilate seemed to be safe in regard to groundwater contamination.