The oncoretrovirus HTLV-I is the etiological agent of adult T cell leukemia (ATL) and tropical spastic paraparesis/HLTV-I associated myelopathy (TSP/HAM). In contrast to the human lentiretroviruses, HIV-I and HIV-2, the causative agents of AIDS, HTLV-I is genetically very stable. We report here the molecular characterization of the envelope gene of an HTLV-I variant present in a TSP/HAM patient from Zaïre, raising the question of its relevance to disease or the ethnic and/or geographical origin of the patient.