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