We have measured the thermal conductivity of argon at temperatures from 302 K down to 150.8 K and at densities up to 25 mol L(-1). The data were obtained with a steady-state method and we employed a guarded parallel-plate apparatus designed especially for investigating the thermal conductivity of fluids in the critical region. To interpret the data in the critical region a scaled crossover equation of state for argon in the critical region has been constructed. Equations for the thermal conductivity and viscosity of argon as a function of density and temperature are presented.