A 65-meter resonant cavity has been constructed in order to experimentally determine the characteristics of long resonant cavities as would be required for a free electron laser1 (FEL). A version using normal incidence mirrors is reported here, and another that includes a grazing incidence mirror is forthcoming. Either version is designed to simulate a FEL operating at 0.5 micron wavelength and is near-concentric with a stability parameter of 0.98. Argon-ion plasma tubes simulate the laser gain that would be provided by a wiggler in an actual FEL. The cavity was constructed on a seismic slab and air turbulence effects were reduced by surrounding the beam with helium in 6" diameter tubes. AlignTept sensitivities are reported and compared to geometrical and diffraction predic-tions3' with good agreement.