The fast-fission cross section of 233Pa has been determined for a fission-neutron spectrum originating from a 233U source that was exposed to thermal reactor neutrons. The of was measured by comparing the fission rate of a known quantity of 233Pa to that of a known thorium sample, using back-to-back fission counters. An average above-threshold value of 775 ± 190 mb has been found, based on a fission cross section of 142 mb for 232Th and an assumed threshold of 0.9 MeV for 233Pa. This value is in very good agreement with published estimated values. The characteristics of the fission counter in the presence of the high beta background resulting from 0.3 to 1.8 Ci of 233Pa were investigated and are presented in the paper.