  We discuss the possibility that mass effects, beyond phase space, may have
substantial influence on the direct determination of $V_{tq}$ in decays of top
quarks. In principle, our results are valid both for singly produced top and
for $t {\bar t}$ pair production. The mass effect is practically irrelevant for
the extraction of $V_{tb}$ from Tevatron data, but it may have implications for
higher energy multi-jet top decay.
