  We evaluate the discovery reach of the ATLAS experiment for down type
isosinglet quarks, $D$, using both their neutral and charged decay channels,
namely the process $pp\to D\bar{D}+X$ with subsequent decays resulting in
$2\ell+2j+E^{miss}_{T}$, $3\ell+2j+E^{miss}_{T}$ and $2\ell+4j$ final states.
The integrated luminosity required for observation of a heavy quark is
estimated for a mass range between 600 and 1000 GeV using the combination of
results from different search channels.
