We have studied the spin resonance at 24 and 56 kMc/sec of cobalt in single crystals of yttrium gallium garnet (YGaG). We find resonances associated with four different combinations of site and charge state. Tetrahedral Co 2+ has the M 2 ground term with the tetragonal splitting parameter 2J9= -36 cm -1 . Octahedral Co 2+ has the 4 7\ ground term with the trigonal splitting A t = +650 cm -1 . It is seen in otherwise pure YGaG, as well as in crystals counterdoped with Si 4+ . Tetrahedral Co 3+ has the 5 E ground term, with Z>^14 cm -1 , |»| =0.66 cm -1 . This value of a, the fourth-order cubic field-splitting parameter, is exceptionally large and makes possible strong magnetic-dipole transitions between states with S z~± :2. Electric-dipole transitions are also observed between these states. Octahedral Co 4+ has the strong-field 2 Ti ground term, with A* = +1000 cm -1 ; its spectrum is very weak relative to Co 3+ , even in crystals counterdoped with Ca 2+ . The absence of a resonance from octahedral Co 3+ implies that this ion has the strong-field l A i ground term. Cobalt does not appear to enter the dodecahedral site in our crystals. Our results are complementary to, and on the whole consistent with, the optical data of Wood and Remeika. We review the information available on crystal fields at the octahedral and tetrahedral sites in garnet, and on the hyperfine structure of cobalt.