It is shown in the present paper how the addition of a moderate amount of carbon black filler to a polychloroprene gum modifies the local strain state and alters the ability of the polymer to strain-crystallize. The study combines mechanical and X-ray diffraction performed during classical mechanical cycling and tensile impact tests. It highlights the fact that the strain modification induced by the filler addition is highly inhomogeneous: the crystallization behavior and the local draw ratio state are affected differently. The partial relaxation of the amorphous fraction by the strain-induced crystallization peviously evidenced in the pure gum is still present and should play a protective role. The effect of temperature on the crystallization correlates with the evolution of stress-strain curves. In particular the role of crystallization in stress-hardening is apparently amplified by the presence of the filler. Preliminary tensile tests reveal a drastic decrease of the induction time necessary for crystallization to develop. This implies a strain amplification effect higher than predicted from mechanical cycling analysis. It is proposed that carbon black particles have no time to relax during the fast stretching period. All these observations point to a synergistic effect between filler addition and strain-induced crystallization.