In a polygamous marriage in Senegal, the husband and his two spouses were infected with HIV-1 group O. This study provides new full-length genome sequences for the two spouses (99SE-MP1299 and 99SE-MP1300) and the 3'-end LTR-tat fragment (6084 bp) for the husband (98SE-42HALD). Phylogenetic tree and diversity plot analysis revealed that the new viruses belong to HIV-1 group O and that they are closely related to each other in a cluster around ANT-70. The intrafamilial transmission occurred at most 6 years ago. The interpatient variability was highest in the envelope region, and in some regions of the envelope the strains from the two spouses do not cluster together anymore. The source of infection was in Cameroon and confirms a slow but continuous spread of HIV-1 group O viruses.