On the assumption that the colors observed on the fracture surfaces of poly(methyl methacrylate) are due to interference in a thin layer of structurally modified material, the peculiar arrangement of the colors can be explained if (a) the mean fracture plane lies centrally within modified layer and (b) the fracture surface is regularly biplanar. Compliance with the first condition would be expected from the mechanical system which obtains during fracture. Interference microscopy shows that the surface topography is in accord with second condition. The occurrence of hyperbolic surface features, due to the initiation of a secondary fracture, introduces only a temporary disturbance in the general pattern. Evidence for the mechanism of formation of the hyperbolas is also found in the interference micrographs.