A simple, single-channel 2 mm microwave interferometer has been developed for plasma electron density measurements on the Joint European Torus (JET) experiment. A long (76 m) plasma-probing beam path in an oversized waveguide enables all sensitive components to be located outside JETs biological shield. A high sensitivity liquid-helium-cooled InSb detector compensates for the high waveguide attenuation ( approximately 65 dB). Detection and digitisation noise limit the minimum resolvable phase change with the interferometer to +or-1/50th of a fringe. Since the instrument measures the electron density integral, integral n_e dL, along a nearly central vertical chord through the plasma, this corresponds to an average electron density change along the chord of only +or-76*10¹⁵ m^-3 for a typical plasma with a 3 m vertical diameter. The maximum measurable electron density is expected to be limited by refraction to approximately 8*10¹⁹ m^-3 ( approximately 230 fringes in the interferometer).