A construction toy system comprised of a plurality of molded plastic girders and connectors. The girder elements of generally square cross section, with interlocking elements extending axially from each end. The interlocking elements each comprise a flange of square configuration, a groove structure of squared configuration adjacent the flange, and a generally cylindrical rod-like extension projecting between the end of the girder body and the groove structure. Connector elements are formed with one or more sockets for receiving interlocking elements of the girders. The sockets have one open side into which the interlocking elements can be inserted with a lateral snap-in action. The flange portions are freely received in a slot formed in the socket, and the groove structure engages and cooperates with opposed locking ribs formed on the sidewalls of the connector socket. Opposed axial grooves in the socket sidewalls engage opposite side portions of the rod-like extension with a snap-in locking action. At least certain of the girder elements include positioning and retaining elements for mounting of wall panels as part of an overall structure formed with girders and connectors. The above described elements are compatible with existing K'NEX construction toy sets utilizing rods instead of girders, enabling hybrid structures to be formed with combinations of girders and rods, for example.