The deoxyribonucleic acid (DNA) is a new “green” photonic material with promising applications in organic photonics. In this paper we report the results obtained in an experimental study of the spatial phase modulation induced by light in DNA – cetyltrimethyl-ammonium chloride (CTMA) – Disperse Red (DR1) films. Using a pump-probe interferometric method, recently introduced by us, the spatial distribution of the optical phase change, its magnitude and its dependence on the incident pump intensity are determined in the investigated DNA-CTMA-DR1 films. Processes responsible for the light-induced phase modulation in DNA-CTMA-DR1 film are briefly discussed and their contribution to the overall phase modulation is estimated. The results are important for nonlinear photonics in applications based on all-optical spatial light modulation.