The retinal binding pocket in rhodopsin accommodates three isomeric states at 77 K: 11-cis (rhodopsin), all-trans (bathorhodopsin), and 9-cis (isorhodopsin) forms. A previous Fourier transform infrared Study of bovine rhodopsin observed an isomer-specific protein band, which appears at 3463, 3487, and 3481 cm(-1) for rhodopsin, bathorhodopsin, and isorhodopsin, respectively [Kandori, H.; Maeda, A. Biochemistry, 1995, 34, 14220-14229]. The present infrared study of the rhodopsin mutants revealed that the band originates from the O-H stretching vibration of threonine at position I IS. The frequency is highly sensitive to various mutations, suggesting that the O-H group of Thrl 18 is located at a crucial position in the interaction with the retinal chromophore. The recent crystallographic structure of rhodopsin indeed showed that the O-H group directly interacts with the 9-methyl group [Palczewski, K.; Kumasaka, T.; Hori, T.; Behnke, C. A.; Motoshima, H.; Fox, B. A.; Le Trong, L; Teller, D. C.; Okada, T.; Stenkamp, R. E.; Yamamoto, M.; Miyano, M. Science 2000, 289, 739-745]. On the basis of the present results and the structure of rhodopsin, isomerspecific interaction and the isomerization process in rhodopsin are discussed.