Simian immunodeficiency viruses from African green monkeys (SIVagm) are the most genetically heterogeneous type of non-human primate lentivirus. To further examine the extent of genetic divergence within the SIVagm type, we generated and sequenced a biologically active proviral DNA clone representing a lentivirus isolated from a distinct African green monkey species (grivet). Overall, this clone (gri-1/lambda II) was highly divergent from previously characterized SIVagm clones. Specifically, the difference between gri-1/lambda II and other SIVagm clones approximated the difference between the caprine arthritis encephalitis virus and visna virus (two biologically and genetically distinct ruminant lentiviruses). Our data suggest that lentiviruses from African green monkeys may have evolved in concert with speciation of the genus and support the concept that SIVagm may be the oldest primate lentivirus type in existence.