We propose a model for the stress softening of isotropic, incompressible rubberlike materials. The model is derived from a micromechanical scheme of a polymeric network reinforced with fine filler particles, idealized as rigid, and connected by two different types of chains: elastic and breakable. The fraction of breakable chains, assigned through an appropriate distribution function, is responsible for the network alteration. This prototypical system is then extended to a three-dimensional model with isotropic stress softening. In order to illustrate this model, we discuss two explicit examples: the homogeneous deformation of uniaxial extension and the inhomogeneous deformation of azimuthal shear. (c) 2006 The Society of Rheology.