Femtosecond fluorescence spectroscopy has been used to investigate the excited-state dynamics of a rhodopsin analog having retinal modified by a flexible eight-membered ring. The fluorescence lifetime was determined to be 60 fs throughout the observed wavelengths. Combined with the previous transient absorption results (Mizukami, T.; Kandori, H.; Shichida, Y.; Chen, A.-H.; Derguini, F.; Caldwell, C. G.; Bigge, C. F.; Nakanishi, K.; Yoshizawa, T. Proc. Natl. Acad. Sci. U.S.A. 1993, 90, 4072-4076), the ultrafast process was interpreted in terms of rapid deformation around the C11-C12 double bond, i.e., isomerization, occurring as fast as vibrational motions of the chromophore. The ultrafast reaction suggests that the isomerization occurs in a coherent manner. On the basis of the present results for the modified rhodopsin, the primary processes in the native rhodopsin are discussed.