We present the first measurement of the ratio of branching fractions R≡B(t→Wb)/B(t→Wq) from pp¯ collisions at s√=1.8TeV. The data set corresponds to 109pb−1 of data recorded by the Collider Detector at Fermilab during the 1992–95 Tevatron run. We measure R=0.94+0.31−0.24(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 on the Cabibbo-Kobayashi-Maskawa quark mixing matrix element |Vtb| under the assumption of three generations and unitarity.