We present a measurement of the ratio of top-quark branching
fractions R = B(t --> Wb)/B(t --> Wq) using lepton-plus-jets and dilepton data
sets with integrated luminosity of ~162 pb-1 collected with the Collider
Detector at Fermilab during Run II of the Tevatron. The measurement is
derived from the relative numbers of tt-bar events with different
multiplicity of identified secondary vertices. We set a lower limit of R >
0.61 at 95% confidence level.