Feline immunodeficiency virus (FIV) is a typical lentivirus that preferentially replicates in feline T lymphoblastoid cells and is the causative agent of a cat disease with features similar to the HIV-induced human AIDS. Its overall genetic organization is similar to human immunodeficiency virus (HIV) but the reduced complexity of the regulatory open reading frames renders FIV closer to ungulate than to primate lentiviruses. On the other hand, FIV infects both CD4+ and CD8+ T lymphocytes as well as feline B lymphocytes and macrophages. In addition, the FIV cellular receptor does not appear to be mostly constituted by the feline CD4 differentiation antigen. Nevertheless, the cat model may provide invaluable insight into the determinants of the immunodeficiency viruses pathogenesis. In addition, this model may help define novel approaches to eliciting protective immunity against HIV.