We present the results of a search for pair production of the supersymmetric partner of the top quark (the top squark t̃1) decaying to a b quark and a chargino χ̃1± with a subsequent χ̃1± decay into a neutralino χ̃10, lepton ℓ, and neutrino ν. Using a data sample corresponding to 2.7fb-1 of integrated luminosity of pp̄ collisions at √s=1.96TeV collected by the CDF II detector, we reconstruct the mass of top squark candidate events and fit the observed mass spectrum to a combination of standard model processes and t̃1t̃̄1 signal. We find no evidence for t̃1t̃̄1 production and set 95% C.L. limits on the masses of the top squark and the neutralino for several values of the chargino mass and the branching ratio B(χ̃1±→χ̃10ℓ±ν). © 2010 The American Physical Society.