From March to July 1989 sero-epidemiological surveys have been carried out in four Chadian towns on representative samples of adults aged 15-44 years in the general population. For the first time, HIV1 virus circulation was shown off with seroprevalence rates varying from 0 to 1.1%. Rates are non-existent for HIV2 and they vary from 0 to 1.6% for HTLV1. These low seroprevalence rates make it difficult to identify special risk factors. There was confirmed a preponderance of heterosexual transmissions of these viruses.