A photomagnetic ultrathin film consisting of an azobenzene, a deoxyribonucleic acid, and Prussian Blue has been fabricated by the Langmuir-Blodgett method. Prussian Blue layers in the films were a two-dimensional structure with the height of 50 A and the average coherence length of 210 A. Reversible photoisomerization of the azobenzenes was observed even in the films. The films exhibit the long-range ferromagnetic order below 4.2 K and moreover, upon photoillumination at 2 K, reversible changes in the magnetization were realized with the value of ca. 1.0%. This photoswitching in the magnetization is due to changes in the electrostatic field induced by photoisomerization. (C) 2007 Elsevier B.V. All rights reserved.