Communication: The rapid formation (and erasure) of a dynamic phase grating is one condition required of materials for dynamic holography. The photochemical nematic-isotropic transition of the side-chain polymer liquid crystal (PLC) shown in the Figure, caused by the trans-cis phstoisomerization of the azobenzene moiety, is reported to cause a dynamic grating with fringe spacing of 1.4 mu m to form in similar to 150 ms.