  We compute the one-loop dispersion relations at finite temperature for
quarks, charged leptons and neutrinos in the Minimal Standard Model. The
dispersion relations are calculated in two different plasma situations: for a
vacuum expectation value $\upsilon$ of the Higgs field $\upsilon \neq 0$
(broken electroweak symmetry) and for $\upsilon=0$ (unbroken electroweak
symmetry). The flavour and chiral non-degeneracy of the quasi-particle spectrum
is studied. Numerical results show that the thermal effective masses for
fermions in the broken phase have a smaller value than those in the unbroken
phase. The temperature dependence of the top quark and electron neutrino
thermal effective masses is also presented. Gauge invariance of one-loop
dispersion relations is studied.
