sigma-Conjugated polysilanes and polygermanes are known to possess interesting linear optical properties as well as unique physical and chemical properties. Unlike pi-conjugated carbon-based polymers these materials are transparent in the visible and strongly absorbing in the near ultraviolet region. Here, the nonlinear optical properties, in particular those for third harmonic generation, for several polygermane homopolymers and a polygermane-polpsilane copolymer are presented.