We have examined a large portion of the valvula cerebelli of the mormyrid fish Gnathonemus petersii for responses related to the three known classes of electroreceptors. Evoked potential and extracellular single-unit records from curarized and non-curarized preparations show that a separate area of the valvula is related to each of the different electrosensory modalities. 2. The area related to ampullary receptors is also strongly affected by mechanical stimulation to the skin. In the mormyromast and Knollenorgan regions, responses to sensory stimuli depend on when they are given in relation to the command to fire the electric organ. In the Knollenorgan region the interaction is quite simple. Responses are completely blocked if the stimulus is given during a brief period when the electric organ discharge occurs. A greater variety of interactions is seen in the mormyromast region. 3. Large areas of the valvula do not appear to be clearly and strongly retated to any of the three electrosensory modalities, suggesting the possibility that much of the structure is not devoted to electrosensory processing.
