  We analyse $\Delta C=2$ transitions in the framework of a minimal extension
of the Standard Model where either a $Q=2/3$ or a $Q=-1/3$ isosinglet quark is
added to the standard quark spectrum. In the case of a $Q=2/3$ isosinglet
quark, it is shown that there is a significant region of parameter space where
$D^0 - \bar D^0$ mixing is sufficiently enhanced to be observed at the next
round of experiments. On the contrary, in the case of a $Q=-1/3$ isosinglet
quark, it is pointed out that obtaining a substancial enhancement of $D^0 -
\bar D^0$ mixing, while complying with the experimental constraints on rare
kaon decays, requires a contrived choice of parameters.
