The density of rotational transitions for a polyatomic molecule is so large that in general many such

transitions are hidden under the Doppler profile, this being a fundamental limit of conventional high

resolution electronic spectroscopy. We present here the first Doppler-free cw two-photon spectrum of a

polyatomic molecule. In the case of benzene, 400 lines are observed of which 300 are due to single rotational

transitions, their spacing being weil below the Doppler profile. The resolution so achieved is 1.5 X 10'.

Benzene is a prototype planar molecule taken to have D •• symmetry in the ground as weil as in the first

excited state. From our ultra-high resolution results it is found that benzene in the excited SI state i8 a

symmetrical rotor to a high degree. A negative inertial defect is found for the excited state. The origin of this

inertial defect is discused.