The relative photosensitivity and the molar extinction coefficient of a highly purified iodopsin (chicken red sensitive cone visual pigment) solubilized in a mixture of 3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonate and phosphatidylcholine (CHAPS-PC) were measured using bovine rhodopsin solubilized in 2% digitonin as a standard and compared with those of chicken and bovine rhodopsins. The photosensitivity obtained (1.08) was close to those of rhodopsins (chicken, 1.04; bovine, 0.99) in CHAPS-PC. The molar extinction coefficient of iodopsin (47,200) was 1.15-1.17 times higher than those of rhodopsins (chicken, 40,500; bovine, 41,200). The oscillator strength of iodopsin (0.60) calculated from the extinction coefficient was nearly identical to that of chicken rhodopsin (0.61), suggesting that the chromophore of iodopsin is similar in configuration to rhodopsin. In contrast, the difference in quantum yield between iodopsin (0.62) and chicken rhodopsin (0.70) suggests that the chromophore-opsin interaction after absorption of a photon by the chromophore may be different.