An anomalous sequence in certain lentiviruses was found to be related to a region in a completely different part of the simian retrovirus type I (SRV-I) and its close relative, the hamster intracisternal A particle (IAP-H18). The segment is not present in the human immunodeficiency virus (HIV), which is also a lentivirus, nor is it found in any one of a dozen other retroviruses whose sequences have been reported. These observations imply that a horizontal transfer of newly acquired genetic information has taken place between an SRV-I-type virus and one of the lentivirus type, and that this event occurred more recently than did the divergence of members of this latter group and HIV. Comparison of the viral nucleic acid sequences that encode these segments revealed the presence of imperfect direct nucleotide repeats resembling the retroviral endonuclease cleavage sites at the 5' and 3' ends of these regions.