Re-examination of the known data on crystalline forms of polyglycine reveals that the crystal modification `polyglycine I' has two different three-dimensional structures depending on the molecular weight. Structural models for both low molecular weight (LMW) and high molecular weight (HMW) polyglycine I crystals are described. In the LMW crystal model, the molecules have an unusual extended conformation generated by alternation of two mirror-symmetrical residual conformations along the chain. The molecules are parallel and each chain forms interpeptide hydrogen bonds with four adjacent chains. The structural model for the HMW crystal represents a composition of twinning crystallites. The crystallites themselves consist of antiparallel enantiomorphous chains united by hydrogen bonds to form rippled sheets. Calculations of the diffraction patterns and packing energy show that these polyglycine I structures have a higher level of conformity with the experimental data than previously suggested models. New insight into the structure of the polyglycine associates opens up the possibility of designing improved silk-like and nylon materials.