To measure the fission cross section of233U as a function of neutron energy, instrumentation with fast pulse rise‐time and short resolving time characteristics is required. The efficient detection of fission‐fragment pulses in the presence of a high alpha background requires these characteristics. A fission detector with an electron collection time of 25 nsec and containing 1 g of233U, and an amplifier with a risetime of 7 nsec and an over‐all gain of 1000 were developed for this measurement. This system, detecting the fast rising current pulse from a parallel plate ionization chamber, is an integral part of the instrumentation used by a group at ORNL engaged in the measurement of the neutron fission and capture cross sections as a function of neutron energy. With this system, the fission cross section of233U was successfully measured over a neutron energy range from 0.4 to 2000 eV.