The concomitant appearance of crystallites and nanocavities under uniaxial strain is investigated by X-ray scattering in a model natural rubber system. The nanocavities appear after crystallization and only when the true stress is above a critical cavitation stress sigma Cav. The presence of crystallites alone does not influence the calculation of the void volume fraction phi void. The nanocavities formed are 20-50 nm in size with a constant aspect ratio. The presence of filler shifts the critical crystallization extension ratio Cry, Cav, and sigma Cav to lower values. The clear correlation between sigma Cav and the crystallinity at the onset of cavitation C(Cav) implies that the crystallites take most of the mechanical loading thus delaying the cavitation in the amorphous phase. Under cyclic loading, nanocavitation is significant only in the first loading and in the successive loadings if the extension ratio is above its maximum historical value. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1125-1138