  The S,T, and U formalism for studying electroweak precision data has recently
been updated to include the effects of new light physics and within this
analysis the latest LEP and SLC data tightly constrain models of physics beyond
the Standard Model. We re-examine the constraints on additional strong and
weakly interacting fermion families in the light of these developments. We
conclude that the precision data favour models with at least one fermion with a
mass below 150GeV.
