A measurement of the mass difference between top and anti-top quarks is presented. In a 4.7 fb−1 data sample of proton–proton collisions at s=7 TeV recorded with the ATLAS detector at the LHC, events consistent with tt¯ production and decay into a single charged lepton final state are reconstructed. For each event, the mass difference between the top and anti-top quark candidate is calculated. A two b -tag requirement is used in order to reduce the background contribution. A maximum likelihood fit to these per-event mass differences yields Δm≡mt−mt¯=0.67±0.61(stat)±0.41(syst) GeV , consistent with CPT invariance.