A finite simple shear deformation of an elastic solid induces unequal normal stresses. This nonlinear phenomenon, known as the Poynting effect, is governed by a universal relation between shear strain and first normal stresses difference, valid for nondissipative isotropic elastic materials. We provide the first experimental evidence that an analog of the Poynting effect exists in aqueous foams where, besides the elastic stress, there are significant viscous or plastic stresses. These results are interpreted in the framework of a constitutive model, derived from a physical description of foam rheology. (c) 2007 The Society of Rheology.