Serum and peripheral blood leukocytes from wild yellow baboons (Papio hamadryas cynocephalus) were tested for the presence of STLV-1-specific antibodies and proviral DNA. Fourteen of 30 sera tested positive by radioimmunoprecipitation assay (RIPA) with HTLV-1. Among 36 DNA samples tested by PCR 15 were positive by double nested PCR for a fragment of the STLV-1 env gene, the most sensitive assay among PCR tests employed. Of 30 animals that were tested both serologically and by PCR in only 1 case were the results discordant (PCR-positive, antibody-negative). The DNA sequences from env (378 bp), pol (212 bp), and LTR (705 bp) were determined for 5, 5, and 2 Mikumi STLV-1 isolates, respectively. The DNA sequences of Mikumi STLV-1 isolates were virtually identical and phylogenetic analysis revealed that they were clearly distinct from previously published baboon STLV-1 sequences, including those STLV-1 isolates presumed to be from yellow baboons. The results of this study suggest that reliable placement of individual STLV-1 within the PTLV-1 phylogeny requires genomic sequences of STLV-1 isolates from wild animals whose taxonomic identity and geographical origin are firmly established and that the LTR is the genomic region of STLV-1 which is the most informative for cladistic analysis of these viruses.