Abstract: Disclosed are the simian T-cell lymphotropic virus type 3 subtype D (STLV-3 subtype D), isolated nucleic acid molecules encoding STLV-3 subtype D polypeptides, such as STLV-3 subtype D envelope, protease, polymerase, tax, rex, and capsid polypeptides, isolated polypeptides encoded by such nucleic acids. Methods are also disclosed for detecting STLV-3 subtype D, for example by detecting a STLV-3 subtype D nucleic acid or polypeptide in the sample. Accordingly, probes, primers, and antibodies for use in detecting STLV-3 subtype D nucleic acids or polypeptides are disclosed. Therapeutic compositions which include isolated nucleic acid molecules encoding a STLV-3 subtype D polypeptides or isolated polypeptides encoded by such nucleic acid molecules are also disclosed.