The121 Sb Mössbauer effect has been utilized to study hyperfine fields in the ferromagnetic alloys NixMnSb for 1≤ x ≥2. For x=2, the crystal structure is of the Cu 2MnA1 (L21) type characteristic of Heusler alloys. As x is reduced, Ni vacancies appear until, for x=1, the ordered C1b structure is obtained. Our results indicate a 121 Sb saturation magnetic hyperfine field of approximately +300 kG with a relatively small dependence on Ni concentration. It is possible to understand this result by using two assumptions: (i) the Ni atoms make only a small contribution to the conduction band and (ii) the Sb hyperfine field samples the average spin polarization of the material. The Mössbauer spectra form a sensitive probe to reveal disorder not easily observed by x‐ray diffraction. A spin‐echo measurement of 123 Sb in NiMnSb gives good agreement with the 121 Sb Mössbauer effect results.