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Both action potentials and evoked potentials were recorded from the tectum opticum of a crotaline snake, Trimeresurus flavoviridis, in response to infra-red stimulation of the facial pit organs. Action potentials from single units were recorded throughout the tectum. Most units responded to contralateral stimulation, while some responded to both ipsi- and contralateral stimulation. Firing patterns were tonic, phasic, or phasic-tonic, depending on the position of the stimulus and the type of unit being recorded. Sensitivity to stimulus movement was observed. All potentials differed from peripheral potentials in firing patterns. Firing frequency was directly proportional to stimulus intensity. Measurements were made of the vertical and horizontal response fields of single units. Background discharge was noted in all units and its nature discussed. The integrative function of the tectum in regard to infra-red perception was also discussed, as well as the possibility of stereoscopic perception.