Using the force sensor of an Atomic Force Microscope, motor forces of the HIV-1 Reverse Transcriptase were measured during active replication of a short DNA transcript. At low load forces the polymerase is mechanically slowed, while at high force (approximately 15 pN) it stalls. From recordings of estimated polymerase turnover velocity versus load force, an approximate force-velocity curve has been constructed. The shape of the curve suggests that load force strongly inhibits the rate-limiting step of the polymerase turnover cycle and that the combined effect of load on all steps involves an effective motion of about 1.6 nm. Earlier results from pre-steady-state kinetics experiments have identified the rate limiting step as the closing of the fingers domain to form a tight catalytic complex. Together these findings indicate that the closing of the fingers domain is a major force-generating step for HIV RT and, by extension, for all DNA polymerase machines.