In this communication, we describe unprecedented conformational changes of poly(L-glutamic acid) (PLGA) assembling at the air-water interface. On the basis of the results from spreading experiments and circular dichroism measurements for the freshly prepared amphiphile, which consists of a PLGA segment (polymerization degree, 40) and two long alkyl chains, it was found that the stable beta-structure of PLGA could be produced specifically by assembling the PLGA segment at the air-water interface and even under an acidic pH, ambient temperature. The conformation of the PLGA segment could be readily controlled by changing the monolayer phase.