We present a first direct measurement of the ratio of branching
fractions R = B(t -> Wb)/B(t -> Wq), obtained by the CDF Collaboration.  We
measure R = 0.99 +/- 0.29(stat+syst) in agreement with the Standard Model
predictions.  We use this result to measure the element |V_tb| of the
Cabbibo-Kobayashi-Maskawa quark mixing matrix.  We obtain |V_tb| = 0.99 +/-
0.15(stat+syst) which translates in the 90% confidence limit |V_tb| \geq 0.8.