Abstract: A synthetic plastic terminal block assembly includes a housing containing a longitudinal through passage in which is mounted a unitary fastening member having a center portion that is connected with the housing, and a pair of end portions resiliently connected for independent displacement relative to the center portion and to the housing, the end portions having opposed inwardly-directed hook-shaped foot portions adapted to extend under the outwardly extending flanges at the upper ends of the legs of a U-shaped mounting rail, thereby to fasten the terminal block to the rail. With this extremely simple design, the terminal block can be mounted downwardly upon the rail from above. The terminal block can be easily disconnected from the rail at either side, and if necessary, the terminal block can be pivoted laterally away from the rail.