In order to study Che conformational behavior of poly(L-glutamic acid) (PGA) at the air/water interface under the influence of compression and expansion forces, PGA was spread on an aqueous acidic subphase and. studied by the Langmuir technique. Several distinct regions of the first compression surface pressure! area (pi/A) isotherm could be identified by the starting and inflection point of the isotherm and by the beginning and the center of the pseudoplateau. The interpretation of the characteristic shape of the pi/A isotherms in the sense of a side-by-side and an interdigitated organization of helical rods is strongly supported by molecular modeling calculations. But the helical surface layer is sensitive to repeated expansion and compression. A well-defined and reaction-kinetic demonstrable change occurs. The reasons for this transformation are discussed. Solidified regions are forming in the layer. Finally, after several compression/expansion cycles a. more rigid monolayer results than that formed by helical rods exclusively. These monolayer ruptures on expansion and clods of PGA molecules were observed using Brewster angle microscopy and ellipsometry.