The retroviral Gag precursor plays an important role in the assembly of virion particles. The capsid (CA) protein of the Gag molecule makes a major contribution to this process. In the crystal structure of the free CA protein of the human immunodeficiency virus type 1 (HIV-1), 11 residues of the C terminus were found to be unstructured, and to date no information exists on the structure of these residues in the context of the Gag precursor molecule. We performed phylogenetic analysis and demonstrated a high degree of conservation of these 11 amino acids. Deletion of this cluster or introduction of various point mutations into these residues resulted in significant impairment of particle infectivity. In this cluster, two putative structural regions were identified, residues that form a hinge region (353-VGGP-356) and those that contribute to an α-helix (357-GHKARVL-363). Overall, mutations in these regions resulted in inhibition of virion production, but mutations in the hinge region demonstrated the most significant reduction. Although all the Gag mutants appeared to have normal Gag-Gag and Gag-RNA interactions, the hinge mutants were characterized by abnormal formation of cytoplasmic Gag complexes. Gag proteins with mutations in the hinge region demonstrated normal membrane association but aberrant rod-like membrane structures. More detailed analysis of these structures in one of the mutants demonstrated abnormal trapped Gag assemblies. These data suggest that the conserved CA C terminus is important for HIV-1 virion assembly and release and define a putative target for drug design geared to inhibit the HIV-1 assembly process.