We have used grazing-incidence X-ray diffraction, X-ray reflectivity, and Brewster-angle microscopy to study an amphiphilic azacrown derivative at the air-water interface. The compound studied has a ring-shaped, flexible core with six alkyl chains attached. As the density is increased, the molecules undergo a phase transition from a face-on structure, with the alkyl tails extending away from the interface, to a thicker edge-on structure, with the tails extending radially away from the core. Concomitant with this rearrangement, the film develops long-range in-plane structural order.