Composite latex particles based on natural rubber latex have been synthesized to yield materials which may be formally regarded as interpenetrating polymer networks and semiinterpenetrating polymer networks. Methyl methacrylate was added to a carefully stabilized natural rubber latex and polymerized in situ using an amine-activated initiator, tert-butyl hydroperoxide. The resulting materials were cast to yield solid sheets. The morphology of the materials was determined both by transmission electron microscopy and by dynamic mechanical analysis. In addition the stress–strain behavior was studied.