EARLY x-ray investigations by Clark (2) and Ott (13) on gutta-percha and balata established the fact that these materials were crystalline in nature even in the unstretched condition. In 1927 Hauser and his co-workers (5) pointed out the similarity of the diagrams obtained on gutta-percha and balata, but it remained for Hopff and Susich (7) to establish definitely the identity of the x-ray diagrams of the two substances. In addition they brought out the interesting fact that, on heating above 60° C., a characteristic change took place in the gutta-percha diagram. Hauser and Susich (6) made a detailed study of this change. They showed that gutta-percha (and hence also balata) underwent a change in crystalline structure at about 68° C. The beta modification, obtained above this temperature, did not transform back into the low-temperature or alpha form, but remained unchanged unless the cooling took place very slowly through the transition point. The last-named authors gave much attention to the securing of well-oriented fiber patterns of both crystalline forms in order to calculate their respective identity periods along the fiber axis. Their results, which are probably the most reliable published to date, give the. alpha form of the hydrocarbon an identity period of 8.78 ± 0.12 A˚. and the beta form an identity period of 4.87 ± 0.07 A˚. More recently Bruni and Natta (1) have examined gutta-percha by means of electron rays and have given a solution for the alpha form in which they find the unit cells to be orthorhombic and to have the axes: a=6.53 A., b=7.68 A˚., and c=9.60 or 4.80 A˚. (fiber period). They find the number of isoprene units (C5H8) per cell to be four.