Crack propagation in an aged and unaged natural rubber vulcanizate was measured using a double cantilever beam, DCB. This type of testing arrangement exhibits a plane strain condition and resulted in crack growth rates two orders of magnitude faster than traditional plane stress testing geometries. To validate the DCB specimens, an investigation into the potential cavitation inside the rubber of the DCB specimens was performed. It was found that no cavitation occurred due to the high speed of the fracture. DCB samples were thermally aged to determine the effect of thermal aging upon the crack growth rate. It was found that crack growth rates increase with thermal aging.