It is proposed that aging is induced by somatic replication of transposable elements (TEs). Most transposable elements in Drosophila reproduce by reverse transcription. Therefore inhibitors of reverse transcriptase were tested for their ability to retard aging in Drosophila melanogaster. Two inhibitors, phosphonoformic acid (PFA) and dideoxyinosine (ddI), were capable of prolonging life span when administered for the first half of the adult life. PFA was investigated further. It also produced a reduction in the rate of decline of behavior. PFA appeared not to act on an infectious agent in these experiments, nor did it alter the food intake. Analogues unable to inhibit RT had no life span prolonging effect at similar concentrations to that of PFA.