Highly reflecting nanostructures of pseudoisocyanine J aggregates in a thin polymer film are prepared. Simultaneous atomic force microscope and reflection microscopic studies show a high degree of correlation between structural and optical properties on submicrometer scales. As a result of strong coupling between light and the aggregate excitonic states, optical modes evolve in the polariton-like reflectance spectra of the aggregate nanostructures which function as self-assembled optical microcavities.