Transversely cut human enamel prisms in tangential longitudinal ground sections were studied by a through-focussing technique. Serial photographs were taken 2u intervals between focusing planes. The prints were analyzed by the superimposition of successive tracings of them and by the making of wax-plate reconstructions of individual prisms. The cross-sectional outlines of prisms varied continuously along their length, and the variation was not entirely random. Over 1/2 the prisms followed undulating courses of periodicity between 4/x and 8/u. This may be responsible for some cross-striations of prisms.