Feline immunodeficiency virus (FIV), an immunosuppressive lentivirus found in cats worldwide, is studied to illuminate mechanisms of lentiviral pathogenesis and to identify key components of protective immunity. During replication, lentiviruses accumulate errors of nucleotide mis-incorporation due to the low-fidelity of reverse transcriptase and recombination between viral variants, resulting in the emergence of a complex viral "quasispecies". In patients infected with HIV-1, env sequences may vary by up to 10% and the detection of quasispecies with greater heterogeneity is associated with higher viral loads and reduced CD4+ T cell numbers , indicating that transmission of more complex quasispecies may lead to disease progression. However, little is known about how FIV evolves as disease progresses, or why some cats develop AIDS rapidly while disease progression is slow in others. The aim of this study was to determine whether disease progression may be governed by viral evolution and to examine the diversity of viral variants emerging following infection with an infectious molecular clone. The FIV env gene encoding the envelope glycoprotein (Env) was examined at early (12 weeks) and late (322 weeks) stages of FIV infection in two groups of cats infected experimentally with the FIV-GL8 molecular clone. Viral variants were detected within quasispecies in cats in the late stages of FIV infection that contained differing amino acid compositions in several variable loops of Env, some of which were identified as determinants of receptor usage and resistance to neutralization. Therefore these results indicate that the FIV env gene evolves during the course of infection, giving rise to variants that resist neutralization and likely lead to disease progression.