  I discuss the properties of pions in ``partially quenched'' theories, i.e.
those in which the valence and sea quark masses, $m_V$ and $m_S$, are
different. I point out that for lattice fermions which retain some chiral
symmetry on the lattice, e.g. staggered fermions, the leading order prediction
of the chiral expansion is that the mass of the pion depends only on $m_V$, and
is independent of $m_S$. This surprising result is shown to receive corrections
from loop effects which are of relative size $m_S \ln m_V$, and which thus
diverge when the valence quark mass vanishes. Using partially quenched chiral
perturbation theory, I calculate the full one-loop correction to the mass and
decay constant of pions composed of two non-degenerate quarks, and suggest
various combinations for which the prediction is independent of the unknown
coefficients of the analytic terms in the chiral Lagrangian. These results can
also be tested with Wilson fermions if one uses a non-perturbative definition
of the quark mass.
