The linear thermal expansion coefficients of the a and c axes of bismuth have been measured from 80°K to 540°K by mechanical means. These same coefficients were determined over a more limited range of temperatures by single crystal and powder x-ray methods. The mechanical and x-ray values did not differ significantly. The expansion coefficient for the c axis was found to be 16.6±0.4×10−6 (C°)−1 at 100°K, 17.3±0.2×10−6 (C°)−1 at 150°K, and 17.6±0.2×10−6 (C°)−1 at 540°K. For the a axis the expansion coefficient was 10.8±0.3×10−6 (C°)−1 at 100°K, 11.6±0.2×10−6 (C°)−1 at 150°K, and 11.8±0.2×10−6 (C°)−1 at 540°K.