  In principle one can test the validity of charge symmetry for parton
distributions by comparing structure functions measured in neutrino and charged
lepton deep inelastic scattering. New experiments make such tests possible;
they provide rather tight upper limits on parton charge symmetry violation
[CSV] for intermediate Bjorken x, but appear to show evidence for CSV effects
at small x. We examine two effects which might account for this experimental
discrepancy: nuclear shadowing corrections for neutrinos, and strange quark
contributions s(x) unequal to sbar(x). We show that neither of these two
corrections removes the experimental discrepancy between the structure
functions. We are therefore forced to consider the possibility of a
surprisingly large CSV effect in the nucleon sea quark distributions.
