The invention relates to a locking mechanism for a cargo door (1, 27) in a fuselage cell (3) of an aircraft. The cargo door (1, 27) is attached by a hinge (5) in the area of a recess (2) to open outwards. The locking mechanism comprises inter alia a number of fuselage cell fitments (9, 41) which are disposed in the area of a loading edge (8) of the cargo door (1, 27), and a corresponding number of cargo door fitments (7, 28, 61) which are fixed in the area of a lower edge (6) of the cargo door (1, 27). In accordance with the invention the cargo door fitments (7, 28, 61) can be brought into positive locking engagement in at least some areas with the fuselage cell fitments (9, 41) and the peripheral loads arising inside the fuselage cell (3)—which cause the main stress on the locking mechanism and which consequently form the basis for the dimensions—are transferred substantially by a preferably flat load transfer surface (15, 48), while the generally smaller radial loads are taken up substantially only by the sliding element (17, 31, 55). The locking of the cargo door (1, 27) is carried out by a movable sliding element (17, 31, 55) which can be secured against accidental displacement by means of an optional security bar (38). Even in the event of the sliding element (17, 31, 55) breaking, the cargo door (1, 27) remains fully secured against uncontrolled opening.