An adjustable manhole cover frame assembly for mounting on a manhole basin upper section (12 ) has an outer ring (22) and an inner ring (20) engaging the outer ring by screw threads and supporting the manhole cover (10). The lower annular surface of the frame assembly rests on a convexly curved surface (36C) of a metallic adapter (36), which in turn rests on the generally flat surface ( 12A) of the manhole basin upper section (12); this allows the orientation of the frame assembly to be adjusted. Also, the frame assembly itself may have parts which allow its upper part to be adjusted for slope when the lower part of the frame assembly has been fixed. Another feature is a hinge mechanism (16) having a hinge arm (74) connecting two pivots (72,76), and having interlocking means (18) which prevent the hinge side of the cover from being raised when the cover is closed.