A study of ferromagnetic resonance at 9300 mc and 3000 mc in single crystals of yttrium iron garnet is described. Cavity perturbation techniques are used with polished spherical samples 9 to 11 mils in diameter. The line widths along the hard axis [100] are found to be 2.3 and 5.7 oe, respectively, at 9300 mc and 3000 mc. It is believed the 2.3 oe is the narrowest resonance line yet observed on spheres of ferromagnetic material at room temperature. Because of the narrow line widths, the nonlinear behavior of these crystals is of particular interest. At 9300 mc the rf field,hcrit, for saturation of the main resonance line is 46 millioersteds. Using Suhl's spin wave theory of nonlinear effects in ferromagnets, the 46 millioersteds (associated with az‐directed spin wave) predicts a spin‐lattice relaxation timeT1of 7.4×10−8sec, in good agreement with the 7.0×10−8sec measured by other independent methods. ThisT1then predicts an ``intrinsic'' line width of 1.54 oersted. Thus the observed line width of these crystals at 9300 mc is within a factor 1.5 of its theoretical limiting value. At 3000 mc, where the subsidiary resonance coincides with the main resonance,hcritis 5.0 millioersteds (associated with spin waves notzdirected). As the temperature is increased toward 340°K, an interesting effect is observed. Thehcritrises quite abruptly; in fact it changes by 10.5 db between 335°K and 340°K. This and other observations are compared with Suhl's nonlinear theory with good results.