The complete nucleotide sequence of an infectious clone of simian immunodeficiency virus of macaques, SIVmac239, has been determined. Virus produced from this molecular clone causes AIDS in rhesus monkeys in a time frame suitable for laboratory investigation. The proviral genome including both long terminal repeats is 10,279 base pairs in length and contains open reading frames for gag, pol, vif, vpr, vpx, tat, rev, and env. The nef gene contains an in-frame premature stop after the 92nd codon. At the nucleotide level, SIVmac239 is closely related to SIVmac251 (98%) and SIVmac142 (96%). It will not be possible to test which features of the viral sequence are critical molecular determinants for the pathogenesis of AIDS.