We analyse observations of infrared crown radiation temperatures over a dense Douglas-fir forest. In classic studies the roughness length for heat (Z_0H) is typically derived by extrapolating the temperature profile in the surface layer towards the surface. Assuming that the surface temperature is given by the observed crown radiation temperature, we obtain that Z_0H is of the same magnitude as the roughness length for momentum (Z_0M) for a data set covering one growing season. For vegetation with complete ground cover typical ratios (Z_0H/Z_0M) of 0.1 are found in the literature. In a previous study (Bosveld, Boundary Layer Meteorology, 1997) it is shown that, in the roughness layer of the present forest, the transport of heat is more efficient than the transport of momentum. Taking into account this different flux-profile relations for heat and momentum a modified roughness length for heat can be defined. It is shown that this new roughness length has the usual value of 0.1 times Z_0M. It is found that the concept of roughness length for heat works well for near neutral and unstable atmospheric conditions. For night time conditions other processes come into play. This is the subject of an acompanying oral presentation. This study indicates that the classical definition of Z_0H is not a true surface parameter but depends also on the flow in the roughness layer.