A Raman spectroscopy study has proven, that on increasing hydrostatic pressure up to similar to 3 GPa gamma-glycine undergoes a phase transition into delta-polymorph, which on decompression down to 0.62 GPa transforms into another form, supposed to be a previously unknown polymorph termed zeta-glycine. The formation of this new form was observed also visually using optical microscopy and is in an agreement with the data from X-ray powder diffraction. On further decompression, delta- and zeta-forms coexisted in the pressure range between 0 and 0.62 GPa. The zeta-form of glycine was preserved at ambient conditions for at least 3 days. (c) 2005 Elsevier B.V. All rights reserved.