The muoniated methyl radical has been detected in a sample of liquid ketene irradiated with positive muons. The muon and proton hyperfine coupling constants were measured at 184 K by transverse field muon spin rotation and avoided muon level-crossing resonance, respectively. After correction for the differing magnetic moments, the union hyperfine constant is only 3% larger than that of the proton. This small isotope effect can be attributed to zero-point motion in the anharmonic C-H (C-Mu) stretch.