Limits on exotic fermions fromE6

We explore the limits that present experimental data and data expected from experiments in the near future can place on the properties of the new, light exotic fermions expected in ${\mathrm{E}}_6$ grand unified theories. We find that the present limit on the width of the $Z$ boson from UA2 can be used to put severe constraints on the masses of the neutral exotic fermions independently of whether they are Dirac or Majorana states. However, stricter limits are obtained in the Dirac case.