We extract the total width of the top quark, Γt, from the partial decay width Γ(t→Wb) measured using the t-channel cross section for single top-quark production and from the branching fraction B(t→Wb) measured in t¯t events using up to 2.3 fb−1 of integrated luminosity collected by the D0 Collaboration at the Tevatron p¯p Collider. The result is Γt=1.99+0.69−0.55 GeV, which translates to a top-quark lifetime of τt=(3.3+1.3−0.9)×10−25 s. Assuming a high mass fourth generation b′ quark and unitarity of the four-generation quark-mixing matrix, we set the first upper limit on |Vtb′|<0.63 at 95% C.L.Received 2 October 2010DOI:https://doi.org/10.1103/PhysRevLett.106.022001© 2011 The American Physical Society