A bstract A search is presented for same-sign top-quark production and down-type heavy quarks of charge −1/3 in events with two isolated leptons ( e or μ ) that have the same electric charge, at least two jets and large missing transverse momentum. The data are selected from pp collisions at $ \sqrt {s} = {7}\;{\text{TeV}} $ recorded by the ATLAS detector and correspond to an integrated luminosity of 1.04 fb −1 . The observed data are consistent with expectations from Standard Model processes. Upper limits are set at 95 % confidence level on the cross section of new sources of same-sign top-quark pair production of 1.4-2.0 pb depending on the assumed mediator mass. Upper limits are also set on the pair-production cross-section for new heavy down-type quarks; a lower limit of 450 GeV is set at 95 % confidence level on the mass of heavy down-type quarks under the assumption that they decay 100 % of the time to W t .