The development of anti-human immunodeficiency virus (anti-HIV) neutralizing antibodies and the evolution of the viral envelope glycoprotein were monitored in rhesus macaques infected with a CCR5-tropic simian/human immunodeficiency virus (SHIV), SHIVSF162P4. Homologous neutralizing antibodies developed within the first month of infection in the majority of animals, and their titers were independent of the extent and duration of viral replication during chronic infection. The appearance of homologous neutralizing antibody responses was preceded by the appearance of amino acid changes in specific variable and conserved regions of gp120. Amino acid changes first appeared in the V1, V2, C2, and V3 regions and subsequently in the C3, V4, and V5 regions. Heterologous neutralizing antibody responses developed over time only in animals with sustained plasma viremia. Within 2 years postinfection the breadth of these responses was as broad as that observed in certain patients infected with HIV type 1 (HIV-1) for over a decade. Despite the development of broad anti-HIV-1 neutralizing antibody responses, viral replication persisted in these animals due to viral escape. Our studies indicate that cross-reactive neutralizing antibodies are elicited in a subset of SHIVSF162P4 infected macaques and that their development requires continuous viral replication for extended periods of time. More importantly, their late appearance does not prevent progression to disease. The availability of an animal model where cross-reactive anti-HIV neutralizing antibodies are developed may facilitate the identification of virologic and immunologic factors conducive to the development of such antibodies.