  Assuming the ${\bar D}^0, D^-, D^-_s$ and $B^+, B^0, B_s^0$ mesons belong to
triplets of SU(3) flavor symmetry, we analyse the form factors in the
semileptonic decays of these mesons. Both quark and meson mass differences are
taken into account. We find a number of relations, in agreement with the
present data as well as with previous analyses, and predict certain ratios of
form factors, not yet measured, most notably the D meson decay constant $f_D =
209 \pm 39$ MeV.
