The thermal conductivity, electrical resistivity, and Seebeck coefficient of heavily phosphorus-doped silicon were measured between 313K and 673K. The cylindrical single-crystal sample had a nominal impurity concentration of 5x10 to the 19th power donors per cu cm. The series comparison method with Armco iron as a standard was used for measuring the thermal conductivity. The relative random error of the thermal conductivity measurements was determined to be 6%. The measured values for the thermal conductivity of the heavily doped sample ranged from 0.562 W/cm K at 373K to 0.265 W/cm K at 673K. The electrical resistivity varied directly with temperature, thus indicating that the lattice scattering of the electrons was dominant in the degenerate semiconductor. The values obtained for the Seebeck coefficient passed through a minimum of -260 mu V/K at approximately 600K. The lattice thermal conductivity was computed from Holland's theory of thermal conductivity. A disorder parameter Gamma of 0.0055, which was 18 times larger than the associated theoretical value for Gamma, was needed to reach agreement between theory and experiment. An electron-phonon interaction term, which was inserted into Holland's analysis, was found to have negligible effect except near 300K where it reduced the calculated thermal conductivity by 35 mW/cm K. (Author).