Elongational flow technique has been used to investigate helix-coil transition of poly(L-glutamic acid) (PGA) induced by pH change. The elongational flow field was generated by a four-roller mill apparatus of the type originally used by G. I. Taylor.(16) The flow-induced nonlocalized birefringence was observed all over the pH range measured. The birefringence, Delta n(176), at a fixed strain rate epsilon = 176 s(-1) decreases rapidly within a narrow pH range with increasing pH. The Delta n(176) value is considered to contain information about both helix content, theta, of the system and the degree of orientation of polymers in the elongational flow field. From the Delta n(176) value, using rotational diffusion coefficient, D-r, the helix content, theta, was estimated at each iso-pH measurement. pH dependence of theta was obtained and was found to correspond well to those determined by optical rotary dispersion (ORD) studies. Even in a hinged-rod conformation, the PGA molecule is expected to be in an extended structure. On the basis of the hydrodynamic considerations about the molecular conformation, it is concluded that the elongational flow technique is useful to investigate conformational transitions of biopolymers.