Spectral sensitivity of the cichlid fish Haplochromis burtoni was measured under both scotopic and photopic conditions using a two-choice, food reward, operant conditioning paradigm. The highest absolute sensitivity (scotopic) is one quantum for every 5 to 50 rods measured at 475 nm (equivalent to a corneal irradiance of 3.8 x 10(6) Q s-1 cm-2). A P500(1) photopigment apparently mediates spectral sensitivity over most of the visible spectrum; microspectrophotometric studies of rods had previously shown them to contain this photopigment. However, the scotopic behavioral action spectrum shows a sensitivity to short wavelength light higher than is consistent with a P500(1) photopigment alone mediating the scotopic visual process. Determinations made under photopic conditions reveal a behavioral action spectrum broader than that found under scotopic conditions and consistent with mediation by interaction of the three known cone types in an opponent processing manner. The calculated photopic threshold value of approximately 10(4) Q s-1 (receptor)-1 is in agreement with results from other species and corresponds to a corneal irradiance of about 7 x 10(10) Q s-1 cm-2.