A simple, single channel 2mm microwave interferometer has been developed for plasma electron density measurements on the Joint European Torus experiment. A long (76m) plasma probing beam path in oversized waveguide enables all sensitive components to be located outside JET's biological shield. A high sensitivity liquid helium cooled InSb detector compensates for the high waveguide attenuation (~65dB). The maximum measurable electron density is expected to be limited by refraction to ~8¥1019 electrons/m3 ~250 fringes in the interferometer). Detection noise limits the minimum resolvable density change to ~ ± 1.6¥1016 e/m3 (± 20 millifringe). The system operates routinely and the maximum JET plasma density to date of ~3¥1019 e/m 3 has been measured without difficulty. In addition plasma density fluctuations which represent a variation of only 0.1% of the maximum density signal can be studied with the interferometer.