Infection of adult mice with lymphocytic choriomeningitis virus (LCMV), a non-cytopathic segmented RNA virus, leads initially to generalized infection, followed by clearance and subsequent lifelong immunity. Indirect evidence has suggested that viral antigens may persist in lymphoid tissues during the phase of immunological memory, but viral genomic RNA has not been detected in previous studies(1,2). During a search for persistent virus in the spleen, we identified LCMV-specific sequences present as DNA by polymerase chain reaction (PCR) in mice over 200 days after infection. In vivo and in vitro studies revealed that reverse transcription of viral RNA into complementary DNA occurred after acute infection of cells of its natural hosts, mouse and hamster, but not of other species and could be inhibited in vitro by azidothymidine (AZT), indicating that this was mediated by endogenous reverse transcriptase activity. These findings reveal a surprising and new pathway of interaction between exogenous RNA viruses and endogenous retroviral, and perhaps other host components, that results in the persistence of virally determined DNA. We speculate that the latter may function in vivo as a form of DNA vaccine.