  The decay $D^0 \to \pi^+ \pi^- \pi^0$ appears to be dominated by $\rho \pi$
states in a configuration of zero total isotopic spin. The spin $J$, parity
$P$, and charge-conjugation eigenvalue $C$ of this final state are therefore
$J^{PC} = 0^{--}$, which cannot be formed of a quark $q$ and antiquark $\bar
q$. If a resonance near $M(D^0)$ dominates the final state, it must be a {\it
hybrid} composed of a quark-antiquark pair and a constituent gluon, or a {\it
tetraquark} $q q \bar q \bar q$. A test for this resonance in electroproduction
is proposed.
