Résumé : The results of a search for pair production of a heavy, top-like quark, t ', in the decay mode t't-'→bW+b-W-→bℓ+νb-ℓ-ν- are presented. The search is performed with a data sample corresponding to an integrated luminosity of 5.0fb -1 in pp collisions at a center-of-mass energy of 7 TeV, collected by the CMS experiment at the LHC. The observed number of events agrees with the expectation from standard model processes, and no evidence of t't-' production is found. Upper limits on the production cross section as a function of t ' mass are presented, and t ' masses below 557GeV/c 2 are excluded at the 95% confidence level.