It was observed that the Tarebia granifera plays a significant role in the control of a Biomphalaria peregrina population introduced in a permanent water body. The densities of this planorbid, which had reached high levels, were notably reduced by two important events: an increase of the water level due to heavy rains, and the introduction of the competitor, whose effectiveness had been tested in a different habitat. Knowledge on the ecology and biology of the competitor and the host in permanent water bodies helped to elaborate the measures of control which led to the reduction of the host densities while those of the competitor increased.