A CO2laser interferometer is used for measuring the average free electron density along a vertical path in Doublet III, a large fusion research tokamak. The CO2laser is used together with a colinear HeNe laser to form a two wavelength interferometer which is capable of measuring plasma fringe shifts which are much less than the fringe shifts due to mechanical vibrations. This diagnostic has been shown to be able to resolve plasma electron fringe shifts smaller than 1/17 of a fringe during the pulsed operation which causes mechanical motion resulting in as many as 10 fringe shifts. The interferometer is operated in the Michelson configuration with a total double path length through the plasma of 6 m. The density resolution then is better than 2×1012cm−3. A density resolution of about 0.5×1012cm−3has been achieved during relatively quiet operation.