Abstract: An adapter for mounting in wallboard-type or other planar ceilings so as to permit the utilization of light fixtures which hitherto could only be used in inverted tee-bar types of suspended ceilings. The adapter includes a plurality of frame members, one for each edge of a ceiling opening, each frame member being configured to have a flange for abutting against the lower ceiling surface and a shoulder thereabove. Corner members for each corner of the adapter have a planar wall and a flange extending from the lower edge thereof. The corner members are transversely adjustable relative to the frame members. In use the ceiling panel is clamped between the flanges of the corner members and those of the frame members and the light fixture is positioned in the opening so that the peripheral lower edge of the fixture rests on the shoulders of the frame members.