The low shear rate slip behavior of monodisperse polystyrene melts was characterized with the parallel plate geometry in a torsional rheometer. It was found that all melts studied exhibited some slip behavior. Tool material of construction showed no appreciable effect. Results were analyzed according to deGennes’ theory of slip, and values of the characteristic slip dimension,b, were calculated for each molecular weight. The slip dimension was found essentially proportional to the molecular weight to the first power. A slider block and spring model is proposed in an attempt to explain slip phenomena in the low shear rate limit.