The superior etendu available 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.