The structure, magnetic properties and magnetostriction of arc-melted polycrystalline Sm 0 . 9 Pr 0 . 1 (Fe 1 - x Mn x ) 2 (0=<x=<0.2) alloys were investigated by microscopy, X-ray diffraction, A.C. initial susceptibility, vibrating sample magnetometer and standard strain gauge techniques. It was found that the matrix consists almost entirely of the MgCu 2 -type cubic Laves phase. The lattice parameter increases, while the Curie temperatures decrease with increasing x. The spontaneous magnetostriction λ 1 1 1 , polycrystalline saturation magnetostriction λ s and saturation magnetization M s decrease slowly for x=<1.0 and abruptly for x>1.0 with increasing Mn content. The changes in the magnetization and magnetostriction can be understood on the basis of the decrease of the 3d-sublattice moment. 1 1PACS number: 75.80.+q, 81.40.-z.