Photochemical reactions of an amphiphilic spiropyran (SP) have been investigated in single-layer LB films. The SP LB films were prepared from a pure water subphase at 30°C. When these LB films were irradiated with UV light at room temperature, SP was converted into photomerocyanine (PMC), followed by the formation of J-aggregate of PMC. In situ AFM observations revealed the presence of large circular domains with diameters of 10–20 μm and height of 4–5 nm before irradiation. Irregularly-shaped structures were evident in the center of most of the domains. On irradiation with UV light, dendritic structures spread radially from the irregularly-shaped structures and developed within the domains. The morphological change was not observed in the domains without the irregularly-shaped structures.