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We present the first measurement of the ratio of branching fractions R≡ B(t→ Wb)/B(t →Wq) from pp̅ collisions at √s =1.8 TeV. The data set corresponds to 109 pb-1 of data recorded by the Collider Detector at Fermilab during the 1992–95 Tevatron run. We measure R=-0.24+0.31(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.