Many tissues are subdivided during development into compartments by boundaries that prevent the free mixing of cells. The cellular mechanisms underlying compartmentalization, although poorly understood, are widely thought to involve compartment-specific cell affinities. On p. 3823, Major and Irvine propose a different model for the dorsoventral compartmentalization of the Drosophila wing in which Notch activation creates a `fence'between the compartments. The researchers show that a stripe of Notch activation is sufficient to create a sharp separation between cell populations, irrespective of their dorsal or ventral identities. The cells at the DV boundary have a characteristic shape, and F-actin accumulates in these cells at the adherens junctions at this interface. Notch activation is both necessary and sufficient for these phenotypes, report the researchers, who conclude that Notch regulates DV compartmentalization by modulating the organization of the actin cytoskeleton to create a boundary fence.