Cis-trans isomerization rates of trans,trans-1,3,5,7 octatetraene (OT) on the first excited singlet state (2 (1)A(g)) potential surface have been obtained as a function of vibrational energy by measuring the fluorescence lifetimes. A stepwise increase in the isomerization rate with increasing energy has been observed, which indicates quantization of the vibrational levels of the transition state for the cis-trans isomerization of a double bond. The energy spacing of 80+/-10 cm(-1) between the first two steps tentatively is assigned to an in-plane bending vibration of the transition state.