Abstract The bound state of an isotropic muonium atom has been detected in both n- and p-type InSb using a high-field μ SR technique. The hyperfine constant obtained for this center ( A = 2464 ± 1 MHz ) is characteristic of deep-level Mu 0 centers at tetrahedral interstitial sites in other cubic semiconductors, which typically ionize above 300 K . In contrast, the Mu 0 center in InSb disappears above about 30 K , which is more characteristic of ionization of a shallow-level impurity. The charge-state dynamics of Mu in InSb is discussed in terms of a deep trap or recombination center, rather than as electron ionization.