  The dimuon and dielectron data from the Tevatron $\bar pp$ collider are used
to probe for heavy quarks, which decay dominantly via flavour changing neutral
current. Depending on whether the $FCNC$ decay occurs at the tree or loop
level, one gets a lower mass limit of 85 or 75 GeV. The former applies to
singlet, vector doublet and mirror type quarks while the latter applies to a
lefthanded quark doublet of the fourth generation.
