In cells infected by HIV-1 mutants lacking a functional Vif protein, APOBEC3G is specifically packaged into progeny virions and then interferes with the process of virus infection. Here, we show that incorporation of APOBEC3G into HIV-1 virions is mediated by the specific interaction of APOBEC3G with the carboxy-terminal nucleocapsid/p6 domain of the Gag polyprotein precursor. As a result, HIV-1 virus-like particles that lack the nucleocapsid domain fail to package APOBEC3G. Surprisingly, RNA was also found to be essential for formation of the nucleocapsid--APOBEC3G complex in vitro, thus raising the possibility that RNA may form a bridge between these two proteins.