The thermal conductivity of β‐ and α‐FeSi2 has been investigated at low temperatures (2 K < T < 90 K) and between room temperature and the transition temperature (1200 K). For undoped β‐FeSi2 results are obtained, which are typical for pure crystals. Doping with Co and Al reduces the lattice thermal conductivity. A band gap (T = 0 K) of 0.9 eV can be calculated from the ambipolar range. The lattice thermal conductivity of α‐FeSi2 is determined by a high concentration of Fe‐vacancies.