The stress-strain behavior of polybutadiene elastomers with various degrees of swelling is investigated for general biaxial strain. Although a number of previous observations have suggested that the classical neoHookean (NH) model describes the uniaxial data of an elastomer in a highly swollen state, the NH model evidently fails to reproduce the biaxial data. This result indicates that the successful fit of the NH model for the uniaxial data of highly swollen elastomers, which has long been recognized, is superficial. We show that the biaxial data in all of the various swollen states including the neat (unswollen) state are satisfactorily described by an Ogden-type strain energy function with a single set of parameters. (C) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 721-728, 2010