  By comparing structure functions measured in neutrino and charged lepton deep
inelastic scattering, one can test the validity of parton charge symmetry. New
experiments allow us to make such tests, which set rather tight upper limits on
parton charge symmetry violation [CSV] for intermediate Bjorken $x$, but which
appear to show sizable CSV effects at small $x$. We show that neither nuclear
shadowing nor contributions from strange and antistrange quark distributions
can account for the experimentally observed difference between the two
structure functions. We are therefore forced to consider the possibility of a
large CSV effect in the nucleon sea quark distributions. We discuss the
consequences of this effect for other observables, and we propose an experiment
which could detect a large CSV component in the nucleon sea.
