The weakly electric fish Apteronotus leptorhynchus produces electric organ discharges which are highly stable in waveform and frequency. Short-term modulations of these discharges, typically displayed during social interactions, are controlled by the prepacemaker nucleus (PPn). Neurons of this thalamic cell group intermingle with cells of the central posterior nucleus (CP) to form a complex called 'CP/PPn'. By employing in vitro tract-tracing techniques, we have, in the present investigation, demonstrated that this complex receives input from the tectum opticum. The tectal input is mediated by varicose fibers forming an elongated stripe at the ventral rim of the CP/PPn. As suggested by retrograde tracing from the CP/PPn, this projection is likely to arise from 'multipolar cells with an ascending axon' previously characterized in a Golgi study [14]. As this tectal cell type has been shown to be predominantly driven by electrosensory stimuli [6], information arising from these cells may be used in controlling modulations of the electric organ discharges.