The nuclear contribution to the specific heat of thulium has been investigated in the temperature range 0.02--0.6 K and in the presence of a magnetic field. The data can be well fitted to C=\ensuremath{\gamma}T+\ensuremath{\alpha}${\mathit{T}}^{3}$+${\mathit{C}}_{\mathit{N}}$, where the first two terms represent the electronic and the lattice contribution, respectively, and ${\mathit{C}}_{\mathit{N}}$ is the nuclear specific heat. Using a two-level Schottky model for ${\mathit{C}}_{\mathit{N}}$, a ground-state energy splitting of 8.2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}6}$ eV and a degeneracy ratio of 1.28 were calculated. Consistent with the estimated very large effective magnetic field (565 T), an external magnetic field up to 14 T was found to have no significant effect on ${\mathit{C}}_{\mathit{N}}$.