A fissian foil-Lexan detector system has been developed to monitor reactor neutrons. It is similar to the system based on counting fission gamma rays but has some advantages; i.e., permanently recorded tracks that can be read any time after exposure; integrated recording; microgram amounts of fissionable material needed; and the elimination of specialized gamma-ray counting equipment. Fission-product damage tracks in the Lexan (or mica) are counted under an optical microscope. For thick foils, fluence is determined from the sensitivity factor of 1.16 × 10−5 tracks/(neutron-barn). Fluence measurements with the two systems are compared for several reactor environments while dose measurements are compared with tissue-equivalent calorimeter values. The use of 232Th to replace 238U as the fissionable isotope for the energy interval of 1.5 to 3 MeV, and of 235U to replace 239Pu for energies <600 ke V, was also investigated. Neptunium is retained as the fissionable material for the energy interval 600 keV to 1.5 MeV and the sulfur-activation detector for energies >3 MeV.