During investigations of mandibular development using a Hilger microfocus X-ray unit, a number of problems arose from the application of the rule of isometry. The aim was (1) construction of a cassette suitable to hold a rodent mandible and photographic plate, (2) orientation of the cassette for correct angulation relative to a lateral view. The cassette was made by fitting a pill box inside another of slightly larger dimensions. A circular window made in the lid of the larger box was covered by tin-foil (0''001 in). Photographic plate was cut to fit the surface of the inner box and a half-mandible placed upon it. The larger lid was replaced and the tin-foil pressed around the mandible, holding it in position. For correct angulation of the teeth relative to the X-ray beam the cassette with mandible was raised at the condylar end until the molars were parallel to the horizontal plane. The angle resulting from this movement was calculated, bisected, and the cassette with mandible lowered to the new angulation. Cassettes were held against a vertically positioned plate on a clamp stand for alignment before exposure.