We have determined the fundamental frequency of the cesium atom 6S1/2-6D3/2 two-photon transition, for the first time, to our knowledge. Moreover, our high-resolution scheme made it possible to address the influence of the nuclear magnetic octupole on the hyperfine structure. We found that the octupole-interaction hyperfine constant deduced from the cesium 6D-level has a value nearly eight times larger than what has been deduced from the 6P-level.