Observation of a theoretically predicted peak in the neoclassical toroidal viscous (NTV) force as a function of toroidal plasma rotation rate is reported. The NTV was generated by applying n = 3 magnetic fields from internal (I-)coils to low plasmas produced with nearly balanced neutral beam injection. Locally, the peak corresponds to a toroidal rotation rate 0 where the radial electric field E r is near zero as determined by radial ion force balance. © 2011 American Institute of Physics.