[en] We use ν/sub μ/-deuteron reactions into exclusive multipion final states to determine the extent to which atmospheric ν interactions can imitate nucleon decay. For decays N→l/sup c/π/sup c/π/sup c/ (l/sup c/π/sup c/π/sup c/π⁰), neutrinos induce background events at the rate 0.07 +- 0.06 (0.03 +- 0.02) per kiloton year. By requiring a multipion final state to be consistent with a rho, ω, or eta, a fivefold reduction of the background is obtained. Thus present-generation experiments are capable of searching for such decays at a lifetime level corresponding to many kiloton years of detector exposure