CCR5 and CD4 are coreceptors for immunodeficiency virus entry into target cells. The gp120 envelope glycoprotein from human immunodeficiency virus strain HIV-1(YU2) bound human CCR5 (CCR5hu) or rhesus macaque CCR5 (CCR5rh) only in the presence of CD4. The gp120 from simian immunodeficiency virus strain SIVmac239 bound CCR5rh without CD4, but CCR5hu remained CD4-dependent. The CD4-independent binding of SIVmac239 gp120 depended on a single amino acid, Asp13, in the CCR5rh amino-terminus. Thus, CCR5-binding moieties on the immunodeficiency virus envelope glycoprotein can be generated by interaction with CD4 or by direct interaction with the CCR5 amino-terminus. These results may have implications for the evolution of receptor use among lentiviruses as well as utility in the development of effective intervention.