The superioretenduavailable from a Fabry–Perot spectrometer is also accessible when used in reflective mode in producing a narrow bandwidth rejection, or ‘‘notch,’’ filter. We consider the instrumental defects and the practical effects involved in realizing a useful device. A simple figure of merit is obtained which allows the Fabry–Perot characteristics to be specified according to the desired performance of the rejection filter. A comparison is made between calculations and a prototype Fabry–Perot notch filter.