  We present the first measurement of the ratio of branching fraction R=
B(t-->wb)/B(t-->Wq) from ppbar collisions at sqrt(s)=1.8 TeV. The data set
corresponds to 109 pb-1 of data recorded by the Collider Detector at Fermilab
during the 1992-1995 Tevatron run. We measure R=0.94+.31-.24 (stat+syst) or
R>0.61 (0.56) at 90 (95) %C.L., in agreement with the standard model
predictions. This measurement yields a limit of the Cabibbo-Kobayashi-Maskawa
quark mixing matrix element Vtb under the assumption of three generation
unitarity.
