The fast-fission cross section of233Pa has been determined for a fission-neutron spectrum originating from a233U source that was exposed to thermal reactor neutrons. The ofwas measured by comparing the fission rate of a known quantity of233Pa 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 for232Th and an assumed threshold of 0.9 MeV for233Pa. 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 of233Pa were investigated and are presented in the paper.