The anisotropic thermal expansion of manganese diboride has been determined with neutron diffraction over the temperature range 5–298 K. The variation of lattice parameter with temperature is defined by the equationsa = 3.0044 + 4.9 × 10−6 Tc = 3.0358 − 2 × 10−6 T + 7 × 10−8 T2. The c parameter is found to contract initially, passing through a minimum at 14 K.