  We investigate the single production and decay of charge -1/3, weak
isosinglet vectorlike exotic $D$ quarks in string inspired $E_6$ theories at
future ep colliders; THERA with $\sqrt s$=1 TeV, L=40 $pb^{-1}$ and CERN Large
Hadron Electron Collider (LHeC) with $\sqrt s$=1.4 TeV, $L=10^4$ $pb^{-1}$. We
found that an analysis of the decay modes of $D$ should probe the mass ranges
of 100-450 GeV and 100-1200 GeV at the center of mass energies, 1 and 1.4 TeV,
respectively.
