The thermosensitivity of biodegradable and non-toxic amphiphilic polymer derived from a naturally occurring polypeptide and a derivative of amino acid was first reported. The amphiphilic polymer consisted of poly(gamma-glutamic acid) (gamma-PGA) as a hydrophilic backbone, and L-phenylalanine ethyl ester (L-PAE) as a hydrophobic branch. Poly(gamma-glutamic acid)-graft-L-phenylalanine (gamma-PGA-graft-L-PAE) with grafting degrees of 749% were prepared by varying the content of a water-soluble carbodiimide (WSC). gamma-PGA-graft-L-PAE with a grafting degree of 49% exhibited thermoresponsive phase transition behavior in an aqueous solution at around 80 degrees C. The copolymers with grafting degrees in the range of 30-49% showed thermoresponsive properties in NaCl solution. A clouding temperature (T-cloud) could be adjusted by changing the polymer concentration and/or NaCl concentration. The thermoresponsive behavior was reversible. (C) 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 4823-4828, 2012