Photoinduced molecular dynamics of a Langmuir film of an azobenzene dye is investigated by means of optical second harmonic generation. The dynamics is induced by turning on and off irradiation with monochromatic light at a wavelength in the dye absorption range. We find different dynamical behaviors depending on the surface pressure of the film. At the lowest pressures, we observe a fully reversible photoinduced dynamics which is most likely related to cis-trans photoisomerization. However, the measured relaxation rate of the dye monolayer on the water surface is somewhat faster than the reported cis-trans rate of the same dye in dilute solutions, probably reflecting the effect of dye-dye or dye-water intermolecular interactions. At the highest pressures, we observe an irreversible or slowly reversible dynamics of still unidentified nature. These results, although still very preliminary, point to a strong influence of the molecular state of aggregation on the photoisomerization processes and possibly also on the related photoinduced reorientation phenomena, in accordance with previous observations on the same dye in Langmuir-Blodgett multilayer films.