  Isosinglet vector-like quarks are predicted by some unified theories of
electroweak and strong interactions. We study hadron collider signatures for
the production and decay of isosinglet vector-like quarks with charge -1/3.
Previous analyses of Run I data from the Fermilab Tevatron are used to set
lower limits of 100--200 GeV/c^2 on the mass of such quarks, depending on
assumptions about mixing with Standard Model quarks and the mass of the Higgs
boson. For future Tevatron data (E_{c.m.} = 1.96 TeV) the corresponding mass
range is (100-270, 100-320) GeV/c^2 for (1, 10) fb^{-1}. At the CERN Large
Hadron Collider (LHC) (E_{c.m.} = 14 TeV, 100 fb^{-1}), an analysis of
flavor-changing neutral-current decay modes should probe an $h$ quark mass
range of 100-1100 GeV/c^2.
