Reverse transcriptases (RTs) are multidomain enzymes of variable architecture that couple both RNA- and DNA-directed DNA polymerase activities with an RNase H activity specific for an RNA:DNA hybrid in order to replicate the single-stranded RNA genome of the retrovirus. Previous structural work has been reported for the heterodimeric HIV-1 and HIV-2 RTs. We now report the first crystal structure of the full-length Moloney murine leukemia virus (MMLV) RT at 3.0 A resolution. The structure reveals a clamp-shaped molecule resulting from the relative positions of the thumb, connection, and RNase H domains that is strikingly different from the HIV-1 RT and provides the first example of a monomeric reverse transcriptase. A comparative analysis with related DNA polymerases suggests a unique trajectory for the template-primer exiting the polymerase active site and provides insights regarding processive DNA synthesis by MMLV RT.