The ventral eye of Limulus contains only one type of photoreceptor. Behaviors mediated by the ventral eye provide an unambiguous representation of the function of that single-receptor type. Such behaviors can be compared with the results of acute, single-cell investigations to assay for the contributions of candidate neural codes in the regulation of behavior (cf. Uttal, 1973). Using an unconditioned tail movement as the response, the psychophysical spectral sensitivity function mediated by the ventral eye of Limulus was measured. This psychophysical function peaked at 525 nm and showed evidence of strong absorption by the cuticle in the short-wavelength portion of the spectrum. Under the conditions of the present experiment, the threshold was 4.5 quanta absorbed per receptor per msec at 525 nm. The spectral transmission of the ventral eye cuticle was also measured. After correction for cuticle absorption, the psychophysical spectral sensitivity function was compared with previously reported spectral sensitivity functions obtained either from electrophysiologic (Millecchia, Bradbury, and Mauro, 1966; Nolte and Brown, 1970) or from microspectrophotometric (Murry, 1966) recordings from single, isolated ventral eye photoreceptor cells. All three functions exhibit a sensitivity peak near 525 nm; the corrected psychophysical and microspectrophotometric functions both display a second peak near 425 nm. A second experiment confirmed the reliability and validity of the 425-nm peak. The coding implications of these findings were explored. A preliminary finding is that, in dichromatic or trichromatic visual systems, two-peaked receptor spectral sensitivity functions produce central, opponent response systems that are qualitatively the same as those produced by single-peaked receptors.