  Very little is known about the mass spectrum of fermions in the standard
model. In this talk, I point out that in the simplest extension of the standard
model, in which all right-handed fields are singlets, it is quite possible that
a fourth-generation neutrino will be heavier that its charged counterpart. The
latter will then be very long-lived. Three plausible values of the mixing angle
give very different phenomenology; the charged lepton will either decay at the
vertex (albeit into a massless neutrino), away from the vertex but within the
detector, or will pass completely through the detector. In the latter case,
then if the detectors at the LHC and SSC have the ability to determine the
velocity of these particles, they will look just like muons and be missed. In
the extreme case in which the mixing angle is only generated by Planck scale
effects (it can't be zero for cosmological reasons), not only is detection
possible, but collection for further study may be possible. Finally, in an
alternative model in which the fourth generation leptons form an isodoublet,
the possible decay of the charged lepton into $\tau Z$ offers a promising,
nearly background free, method of detection.
