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 1.16×10−5tracks/(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 of232Th to replace238U as the fissionable isotope for the energy interval of 1.5 to 3 MeV, and of235U to replace239Pu for energies3 MeV.