Nucleocapsid protein NCp7 of human immunodeficiency virus type 1 (HIV-1) is a small basic nucleic acid binding protein containing two zinc fingers of the form (CX2CX4HX4C) and is present at about 2,000 copies inside the viral core. NCp7 molecules are tightly associated with the genomic RNA dimer to form the nucleocapsid, which also includes reverse transcriptase and integrase proteins. In vitro, NCp7 has been shown to bind specifically to HIV-1 RNA, inducing NCp7-NCp7 interactions. In the viral context, mutagenesis of amino acid residues in the zinc finger domains showed that NCp7 is responsible for the specific incorporation of genomic RNA into virions and is necessary for correct virion assembly and maturation. In this work, we investigated the consequences of mutating conserved basic residues in the N-terminal region that precedes the first zinc finger. Two of the mutants were poorly infectious and showed only limited, though significant, defects in RNA encapsidation and viral protein maturation. Electron microscopy, together with sucrose gradient analysis, revealed defects in particle core structure and heterogeneity among mutant virions. These defects were associated with strong reduction of proviral DNA synthesis and stability in newly infected cells. Taken together, these data show multiple and probably interdependent implications for the NCp7 protein in both early and late phases of the HIV-1 replicative cycle and emphasize it as a target for antiviral drug development.