As the regulation of changes in cellular behaviour, such as cell adhesion and motility, has an important role in growth, development and tissue homeostasis, there is much interest in identifying the genes that control these processes. One candidate molecule, Rap1, has been implicated in the regulation of adhesion and motility in cell culture, and here, Christopher R. L. Thompson and colleagues (p. 335) analyse its role in multicellular development. They generate a Dictyostelium mutant in which a gene encoding RapGAPB is disrupted, and establish that RapGAPB is required to regulate the levels of Rap1. The authors next show that both normal morphogenesis and the correct patterning of specific subtypes of prestalk cells require expression of rapGAPB and the regulation of Rap1 activity. How does Rap1 exert its effects? By comparing the adhesive properties of prestalk and prespore cells in the wild-type and mutant structures, the authors demonstrate aberrant cell adhesion in rapGAPB-null mutant cells. These results establish a role for Rap1 in regulating differential cell adhesion, cell-type-specific patterning and morphogenesis in Dictyostelium, and, furthermore, define a system in which to study the cell and developmental biology of Rap1 regulation and its role in cell adhesion and pattern formation.