While studies were being performed on the electrical properties of frog muscle, trematode larvae suddenly appeared in the muscles of all the frogs that could be obtained. Further studies using these infected muscles showed that the presence of the parasites had considerably upset the physiological activity of these muscles. The sodium and potassium ion concentrations of both muscle and plasma from infected frogs were found to be considerably higher than values reported for normal frogs. The tendency for the recorded potentials, which were initially low, to reach normal values when the infected muscles were kept in Ringer's solution, plus the increased ionic concentrations found in the infected frogs, indicate that the parasites did not have a direct effect on the muscle fibers. In some manner the parasites seem to have considerably upset the water balance mechanisms of the frog.