The thermal conductivities of gadolinium, terbium, and holmium single crystals have been measured as a function of temperature from 5° to 300°K. The steady‐state heat flow method was used. For each element measurements were taken in the ⟨112¯0⟩ (aaxis) and in the ⟨0001⟩ (caxis) directions of the hcp crystal structure. Electrical resistivity measurements were made on the same samples to obtain Lorenz functions. Anomalies near magnetic transition temperatures are interpreted in terms of magnetic superzones and spin‐disorder scattering. High‐temperature anisotropy is interpreted in terms of Fermi surface anisotropy. Lorenz numbers appreciably larger thanL0suggest that, in addition to electrons, phonons and magnons can carry heat.