The hyperfine structure of the 4.5-hour metastable state of indium-115 has been studied by the method of atomic beam magnetic resonance. The values found for the hyperfine interaction constants are −903.5 ± 1.1 and −95.973 ± 0.010 Mc/sec in the 2P1/2 and 2P3/2 electronic states respectively. Neglecting a possible hyperfine anomaly, these correspond to a nuclear magnetic moment for In115m of −0.24371 ± 0.00005 nuclear magnetons. The construction of the atomic beam apparatus, recently completed at McMaster University, is also described.