  In any EAS array, the rejection of events with shower cores outside the
detector boundaries is of great importance. A large difference between the true
and the reconstructed shower core positions may lead to a systematic
miscalculation of some shower characteristics. Moreover, an accurate
determination of the shower core position for selected internal events is
important to reconstruct the primary direction using conical fits to the shower
front, improving the detector angular resolution, or to performe an efficient
gamma/hadron discrimination. In this paper we present a procedure able to
identify and reject showers with cores outside the ARGO-YBJ carpet boundaries.
A comparison of the results for gamma and proton induced showers is reported.
