Singlet quarks beyond the top at the Tevatron?

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 $\text{case}2\mathrm{/}3$ singlet quark, approximately degenerate and mixing with the top quark, would effectively double the standard top signals. A charge $-\text{case}1\mathrm{/}3$ singlet quark mixing with the bottom quark would not affect top signals but would generate excess $Z+$multijet events with a $b$-tag.