We have constructed an expression plasmid containing the portion of the Moloney murine leukemia virus genome encoding the reverse transcriptase (RT). When introduced into Escherichia coli this plasmid induces the synthesis of a 70-kDa protein. The RT made in E. coli differs from the viral protein only in that there are two new amino acids, methionine and glycine, substituted for the threonine found at the N terminus of the viral enzyme. Approximately half of the E. coli synthesized RT enzyme is soluble in cell extracts. This protein is active in an RT assay, and like the enzyme purified from virions, is more active in the presence of Mn2+ than Mg2+. We have also constructed a plasmid that induces the synthesis of an RT-integration protein fusion.