  We consider the dispersion relations of fermions that propagate in the
background of a scalar Bose-Einstein condensate. Some illustrative examples are
discussed using simple Yukawa-type coupling models between the fermions and the
scalar fields. The dispersion relations are determined explicitly in those
cases, to the lowest order. The method also allows to determine the corrections
to the dispersion relations due to the interactions with the excitations of the
Bose-Einstein condensate. Possible applications of the results to the case of
neutrinos are indicated.
