A new system for evaluating the responses of miracidia to chemostimulants is described. The apparatus consists of a translucent plastic block with a center well and a hole in the edge leading to the well. One end of a glass tube, covered with a dialysis membrane, was inserted into the hole. Experimental solutions to be tested were put into the tube and Schistosoma mansoni miracidial behavior was observed in the well on the other side of the permeable membrane. Miracidia were released near the membrane; those which contacted the membrane were scored as to whether they returned (contact with return) or did not return (contact without return) before leaving the field of view. Materials eliciting significantly more contact with return responses than did controls were considered to be stimulatory. In this assay system, snail (Biomphalaria glabrata) conditioned water elicited 75% contact with return as compared to 8% for well water control (P less than 0.05). Tracings from motion pictures showed swimming behavior of miracidia toward snail-conditioned water to be different from behavior toward well water controls. This system permits generation of dilution response curves for chemicals and provides generally quantitative results.