Muon-number-violating hyperon decays are discussed, first from a phenomenological point of　view and then considering some possible sources of muon-numberviolation. Weshow that branching　ratios as large as of the order of 10~6 for the decays 1+-*py*e± and E°^-Afife± cannot be　ruled out. We note, however, that such branching ratios are unlikely, since they would require　muon-number-violating interactions of very special structure: