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Magnetic hyperfine constants have long been used as probes of chemical bonding, since they are averages of various functions of the valence electrons near the spinning nucleus. Traditionally, for gas-phase molecules these interactions have been determined from the analysis of splittings in the pure rotational spectra, but the reactive nature of transition metal hydrides (TMH) has precluded them from this experimental approach. Here we report on the first recording, and subsequent analysis, of the microwave spectra of a TMH. Numerous transitions between the A-doublet levels in the $X2\Delta 3/2$ state of NiH have been measured using the microwave optical double resonance technique. The data was reduced using an effective hamiltonian approach and magnetic hyperfine parameters were determined. The fine structure parameters were held fixed to those obtained from the recent LMR1 values.