In this paper, we report the synthesis and the monolayer behavior (surface pressure and surface potential) of a new amphiphilic coumarin dye, 7-aminocoumarin-4-acetic acid octadecylamide (ACO), at the air-water interface. The spectroscopic characteristics (UV-vis, fluorescence, and fluorescence imaging) of pure and mixed films (1:20, ACO/stearic acid and ACO/oleic acid) at the air-water interface as well as Langmuir-Blodgett (LB) films have been investigated and compared with the spectroscopic characteristics of ACO in solution. These experiments provide evidence of aggregate formation during compression of the monolayer at the air-water interface. Surface pressure dependent in situ fluorescence imaging confirms that the fluorescent quenching of the dye with increasing surface pressure originates as a result of formation of nonfluorescent aggregates. Atomic force microscopy imaging of a pure ACO LB film shows that the size of these aggregates is in the nanometer scale. This work provides information that ACO forms a stable monolayer and may be utilized as an efficient molecular probe for monolayer studies.