A calculation is presented of the elastic constant C 33 of a graphite crystal as a function of temperature up to 2500 K, taking into account the anharmonic contribution and the changes in interlayer interactions due to the large lattice thermal expansion. Parametric variations in the theory show that the anharmonic contribution to C 33 depends principally on the parameter ( ∂ 2C 33 ∂e 2 zz ) Comparison of theoretical results with the experimental data, which is mainly from neutron scattering experiments, shows that the data can be accounted for if ( ∂ 2C 33 ∂e 2 zz ) lies in the range 7–10 × 10 13 dynes/cm 2. A theoretical estimate of ( ∂ 2C 33 ∂e 2 zz ) based on Lennard-Jones potentials between atoms in adjacent basal planes gives a value of 9·07 × 10 13 dynes/cm 2.