An experimental study is made of magnetically insulated electron flow in a cylindrical thermionic diode. Emphasis is laid on measurements of plasma waves and leakage current in the insulated state. It is found that over a wide range of conditions the leakage current can be represented byI=[(0.84×10−6&xgr;AcV32)/deff] exp(4015x@B|/&xgr;) Amps, whereAcis the cathode area,Vthe anode–cathode voltage,deffis an effective interelectrode gap (m), andx*is the thickness of the Brillouin layer in contact with the cathode (m). Here &xgr; is a geometrical factor equal to unity in the present nearly planar geometry, increasing by 3 as the cathode radius becomes much less than the anode radius, as is the case in prior magnetron data also fitted by this formula. Unstable electrostatic, transverse magnetic (TM) waves are observed at frequencies predominantly below the electron‐cyclotron frequency with wavelengths equal to the &pgr;d/n, whered=diode diameter,n=1,2,3... . The waves are believed to be driven unstable by a resistive wall instability. The correlation between the leakage current and the wave amplitude is discussed.