Large contributions to the anomalous magnetic moment of the muon in supersymmetric theories

We show that, in supersymmetric theories beyond the Minimal Supersymmetric Standard Model, the anomalous magnetic moment of the muon can receive large contributions at one-loop level, of O(m(tau)m(mu)), rather than O(m(mu)(2)). We evaluate all such contributions and show that, for low supersymmetric masses, they already exceed the value measured by the Brookhaven E821 experiment. These contributions put more stringent mass constraints on the parameters of the model and can distinguish it from the same process in the MSSM.