We isolated a divergent simian T-lymphotropic virus (STLV) (strain PP1664) from a wild-caught African bonobo (pygmy chimpanzee, Pan paniscus). Molecular and phylogenetic characterization of this virus show that it reliably separates from the two well-established primate T-lymphotropic virus types, HTLV-I/STLV-I (PTLV-I) and PTLV-II, and from a third type isolated from an African-born Papio hamadryas and designated by us as PTLV-L. Four of eight bonobos kept at the Antwerp Zoo, Belgium, showed an aberrant PTLV serology. We amplified and sequenced a 709 bp PTLV proviral tax/rex fragment from one of the reactive bonobos. It differs by about 25 % from the homologous nucleotide sequences of PTLV-I and PTLV-L and by about 17 % from PTLV-II. This is comparable to the differences among the three known types. Including the most divergent STLV-I strains sequenced to date, for example, strain PHSu1 sequenced here, the divergence in this region within PTLV-I is less than 11 % and within PTLV-II less than 4%. Although very divergent, this new bonobo STLV is the closest well-characterized simian relative of HTLV-II, raising the possibility of very divergent new HTLV strains. Our results show that the number of PTLV types should be considered open and that the variety of indigenous viruses in the PTLV group is greatest in Africa. Thus, as for the other primate retroviruses HIV and SIV, PTLV most probably has its origins in Africa.