The nucleotide sequences of the envelope genes of an African and a North American acquired immunodeficiency syndrome (AIDS) viral isolate have been determined. When their deduced amino acid sequences were aligned with the envelopes of the lymphoadenopathy and AIDS-associated retrovirus isolates, conserved and divergent regions were readily identified. Hypervariable stretches of 28-74 amino acids, exhibiting 20-30% amino acid identity at each position and characterized by reciprocal insertions and deletions, were confined to the gp120 external envelope protein. The origin and clinical implications of the AIDS viral env gene diversity are discussed.