We report interferometric observations of the classical galactic Cepheid (zetz) Gem with FLUOR (Fiber Linked Unit for Optical Recombination), installed at the IOTA (Infrared Optical Telescope Array) interferometer. Thanks to the high precision of the visibility measurements with FLUOR, it has been possible to estimate its mean uniform disk angular diameter to a relative precision of 5% ((theta) ud equals 1.98 +/- 0.09 mas). Variations of the angular diameter of (zetz) Gem were marginally detected, with an amplitude of (Delta) (theta) ud equals 0.38 +/- 0.23 mas, but further observations should allow a more precise estimation of this value. The feasibility of the observation of Cepheids with the VLTI is also evaluated. When in operation, the VLTI will allow the precise measurement of the angular diameters of a large number of Cepheids. The zero point of the period- luminosity relation could then be set with a precision of less than 0.1 mag.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.