  We study if the neutrino mixing parameters suggested by the atmospheric
neutrino anomaly imply chemical equilibrium between mu- and tau-flavored
leptons in a supernova (SN) core. The initial flavor-conversion rate would
indeed be fast if the nu_mu-nu_tau-mixing angle were not suppressed by
second-order refractive effects. The neutrino diffusion coefficients are
different for nu_mu, anti-nu_mu, nu_tau and anti-nu_tau so that neutrino
transport will create a net mu and tau lepton number density. This will
typically lead to a situation where the usual first-order refractive effects
dominate, further suppressing the rate of flavor conversion. Altogether,
neutrino refraction has the nontrivial consequence of guaranteeing the separate
conservation of e, mu, and tau lepton number in a SN core on the infall and
cooling time scales, even when neutrino mixing angles are large.
