One theoretical and two experimental methods are described for finding the accelerating field in the M.I.T. linear accelerator cavity, in terms of the input power. One of the experimental methods is based on measuring the power leaking out through a calibrated hole in an end wall closing the cavity, the hole being located where the field strength is to be determined. The other method is based on the perturbation of the resonant frequency of the cavity by a small conducting sphere located on the axis. All three methods of determination check satisfactorily. In terms of the resulting relations, we discuss the probable field actually existing in the accelerator under operating conditions.