Superconductings cavities for linear accelerators applications at 4.5 GHz (C Band) have been constructed and tested. The Niobium cavities were obtained by electron beam welding of half cavities previously formed pressing Niobium sheets in a hydraulic press. In such cavities, operating in TM010mode, accelerating field of 26.5 MV/m and peak surface field of 50 MV/m were obtained at a power level of 30 W/m. The best quality factor obtained at this field level is Q0= 2 × 109at a temperature of 1.8°K. The set of tests on C-Band cavities shows: (1) our C-Band niobium cavities exhibit quite good R.F. properties analogous to the ones measured at higher frequencies (X-Band); (2) limitations on accelerating fields are related only to R.F. induced magnetic field. The electron loading due to field emission does not limit the maximum achievable field.