  The first evidence for the top quark at the Tevatron may indicate a cross
section higher than the QCD expectation. We consider the possibility that
isosinglet heavy quarks may be contributing to the signal and discuss ways of
testing this possibility. For example, a charge $\case2/3$ singlet quark,
approximately degenerate and mixing with the top quark, would effectively
double the standard top signals. A charge $-\case1/3$ singlet quark mixing with
the bottom quark would not affect top signals but would generate excess
$Z+{}$multijet events with a $b$-tag.
