We have used electron cryo-microscopy and image analysis to examine the native structure of immature, protease-deficient (PR-) and mature, wild-type (WT) Moloney murine leukemia virus (MuLV). Maturational cleavage of the Gag polyprotein by the viral protease is associated with striking morphological changes. The PR- MuLV particles exhibit a rounded central core, which has a characteristic track-like shell on its surface, whereas the WT MuLV cores display a polygonal surface with loss of the track-like feature. The pleomorphic shape and inability to refine unique orientation angles suggest that neither the PR- nor the WT MuLV adheres to strict icosahedral symmetry. Nevertheless, the PR- MuLV particles do exhibit paracrystalline order with a spacing between Gag molecules of approximately 45 A and a length of approximately 200 A. Because of the pleomorphic shape and paracrystalline packing of the Gag-RNA complexes, we raise the possibility that assembly of MuLV is driven by protein-RNA, as well as protein-protein, interactions. The maturation process involves a dramatic reorganization of the packing arrangements within the ribonucleoprotein core with disordering and loosening of the individual protein components.