We report a measurement of the lifetime of the 2 S&&2 state in one-electron krypton and a direct determination of the branching ratio for the M1 decay of this state. The branching-ratio measurement provides a sensitive test of the relativistic corrections to the decay rate. The lifetime measurement yields (36.8+1.4) ps and the branching ratio is found to be 0.356(15), both in good agreement with the theoretical values of 37.008 ps and 0.3643, respectively. From these measurements we also deduce the corresponding magnetic-dipole decay rate AM&=9. 68 (55)X10's ' and the two-photon decay rate =1.750(78) X10' s PACS number(s): 32.70.Fw, 31.30.Jv, 31.10.+z