The thermal conductivity and the Seebeck coefficient of two undoped InP crystals were measured between 300 and 800\ifmmode^\circ\else\textdegree\fi{}K. Computed values of the Seebeck coefficient, assuming pure polar scattering, are in excellent agreement with the experimental data. The electronic thermal conductivity was found to be negligible in InP. The interpretation of the lattice thermal resistivity provides strong evidence for the influence of optical mode scattering on the thermal resistivity of III-V compounds, which was postulated earlier by the authors.