Poly(beta-phenylpropyl L-aspartate) exhibits a novel helix-helix transition in the solid state at around 160 degrees C. The transition takes place reversibly in a narrow temperature range during a heating and cooling cycle, indicating that it is essentially of a first order. A combined use of circular dichroism, X-ray, and C-13 CP/MAS NMR measurements revealed that the transition is due to an interconversion of the helical screw sense from a right-handed alpha-helix to a left-handed helix. An unwinding and rewinding of the helical screw occurs concertedly along the polypeptide backbone without disturbing the uniaxial orientation of the helix axes.