Proteins that contain F-BAR domains, which control the curvature of biological membranes, have been identified in large numbers; in humans, for example, there are 36. However, the specific functions of the individual family members remain unclear. On page 1054, Robert Heath and Robert Insall identify six genes that encode F-BAR proteins in the social amoeba Dictyostelium. The authors characterise single and double knockouts of the genes that encode two of these proteins (MEGAP1 and MEGAP2), and show that mgp1– and mgp1–/2– cells have elevated numbers of contractile vacuoles (CVs) – networks of interconvertible tubules and vesicles that regulate osmolarity in the cytoplasm by pumping excess liquid from the cell. Moreover, the expulsion of fluid from CVs is impaired in all three MEGAP mutants. In cells that express GFP-MEGAP1, the labelled protein localises to the tubules of CVs, but redistributes to the cytoplasm when tubules convert to vesicles in response to hypo-osmotic stress. Thus, MEGAPs appear to stabilise tubules in the CV network, and to enhance liquid expulsion by promoting the vesicle-to-tubule transition, indicating a novel role for F-BAR proteins in osmoregulation.