Two distinct requirements for successful chemical control of Oncomelania nosphora should be recognized, namely, reduction of the prevailing population to a level that would result in isolation of the survivors; and repopulation control directed against the juvenile snails. These two requirements are considered complementary for ultimate success with molluscicides. An experiment, primarily an evaluation of a procedure, has been carried out which entailed three 6-hour applications of dinitro-o-cyclohexylphenol (1 or 2 ppm) in irrigation water at 3-week intervals during the summer. Irrigation flow was interrupted for 18 hours after each application, during which time snail collections were obtained on a time basis (snails per man-hour). Evaluation was based on reduction of the juvenile snail population. Results indicated that repopulation control directed against juvenile O. nosophora is feasible in the irrigation system of Japan if initial reduction of the prevailing adult population is carried out.