Systematic x-ray and electron diffraction studies of polymers of 1,3-butadiene prepared by different anionic catalysts have made it possible for us to identify two types of highly crystalline materials. One of us (G. N.) has already reported that our synthesis was carried out with the aid of different highly selective catalysts. The description of the preparation of these catalysts, which have made it possible for the first time to prepare these new polymers, will be separately published by one of us (G. N.) with L. Porri. Infrared analysis carried out by E. Mantica and D. Morero in our Institute has established that one type of these crystalline polymers obtained with heterogeneous catalysts represents a continuous l,4-trans arrangement of the monomer units. X-ray diffraction has shown that this material is dimorphous, having one crystalline modification that is stable at room temperature and another stable above 60° C. The x-ray melting point of a polymer having a molecular weight of the order of 100,000 is around 130° C. The low-temperature modification has an identity period of 4.9 A along the chain axis and a hexagonal packing with a period of a=4.54 A perpendicular thereto, but the symmetry of the crystals, as a whole, is lower. The high temperature modification seems to have the chains in helicoid form. The other crystalline polymer consists essentially of 1,2-monomer units; infrared studies have shown that more than 90 per cent of its double bonds are of the vinyl type and the residual 10 per cent represents a mixture of cis and trans. The higher the proportion of 1,2-units, the more crystalline is the material. Figure 1 shows the infrared absorption curves of two different typical samples of the product. In this article, we limit ourselves to describing the structure of the latter polymer, which, in our opinion, is the more interesting material and which we shall simply call 1,2-polybutadiene. We shall report on the structure of the two modifications of the l,4-trans-polybutadiene in another paper together with the study of the structure of other crystalline polydienes.