We report measurements of the viscosity of a 0.1% 3He–4He mixture obtained from the damping of second sound at low temperatures and in a magnetic field of 10T. The damping of second sound arises from the bulk viscosity, the surface viscosity and from losses on reflection from the ends of the cavity. From the frequency dependence of the widths of the resonance we obtain the viscosity accurate to 3%. The viscosity is observed to increase when a field is applied and this increase is in agreement with the recent prediction of Hampson et al. who calculated changes in the transport properties of 3He–4He mixtures for temperatures of order the fermi temperature.