In the case of an aircraft aft portion equipped with an engine mounting structure passing across the fuselage, assembly of the aft portion presents problems caused by the moving of the box structure of the mounting structure through lateral openings in the fuselage. In order to overcome these problems, a method of assembling an aircraft aft portion is proposed, in which method the box structure of the engine mounting structure is inserted into the fuselage through a top opening extending from one side of the fuselage to the other across a vertical midplane of the fuselage such that the top opening opens to the top and to the sides of the fuselage.