We report a quantum-chemical study of the resonance Raman spectra of the lowest triplet state of 1,3,5-hexatriene. Optimized structures in T-1 and in the state (T-n) in resonance with the excitation wavelength are obtained at the ab initio CASSCF level of theory. Vibrational force fields of the E-, Z- and P-rotamers in the lowest triplet state are evaluated and the displacement parameters that govern the activity of totally symmetric modes are derived. The vibrational structure of the RR spectra is modelled for do-hexatriene and four deuterated isotopomers. Comparison of simulated and observed spectra shows that the activity of the E-isomer of hexatriene accounts very satisfactorily for the observed bands. The contribution of the Z-form is shown to be negligible in agreement with its population in T-1.