Receptors located in the facial pit organ of certain species of snake signal the presence of prey. Infrared radiation is an effective stimulus suggesting that these receptors may be low-threshold temperature receptors. We recorded from the nerve innervating the pit organ of snakes belonging to the family of Crotalinae while stimulating the receptive area with well-defined optical stimuli. The objective was to determine the sensitivity of these receptors to a wide range (0.400-10.6 µm) of optical stimuli to determine if a temperature-sensitive or photosensitive protein initiated signal transduction. We found that receptors in the pit organ exhibited a unique broad response to a wide range of electromagnetic radiation ranging from the near UV to the infrared. The spectral tuning of these receptors parallels closely the absorption spectra of water and oxyhemoglobin, the predominant chromophore in tissue. Our results support the hypothesis that these are receptors activated by minute temperature changes induced by direct absorption of optical radiation in the thin pit organ membrane.