Neural activity in the optic tectum was compared with activity in the nucleus isthmi (NI) of both goldfish and sunfish with the aim of understanding how the two brain structures interact to process visual information. The two species yielded very similar results. Superficial tectum responds reliably to visual stimulation with topographically organized receptive fields; deep tectum and NI respond to stimulation throughout the field of the contralateral eye and habituate rapidly. Bursts of large-amplitude spiking in NI occur spontaneously and in response to contralateral visual stimulation. These NI bursts correlate with activity bursts across the tectal lobe on the same side, especially in the deeper layers. NI bursts may also synchronize with spiking activity in deep tectum. Trains of small-amplitude spikes in NI can be elicited by both ipsilateral and contralateral stimulation, but are not reflected in tectal activity. Simultaneous recordings from two sites in one NI were almost identical, suggesting that NI operates as a functional unit, broadcasting the same message across the ipsilateral tectal lobe.