Seven-pass transmembrane cadherins (7-TM cadherins) play pleiotropic roles in epithelial planar cell polarity, shaping dendritic arbors and in axonal outgrowth. In contrast to their role in planar polarity, how 7-TM cadherins control dendritic and axonal outgrowth at the molecular level is largely unknown. Therefore, we performed extensive structure-function analysis of the Drosophila 7-TM cadherin Flamingo (Fmi) and investigated the activities of individual mutant forms mostly in dendritogenesis of dendritic arborization (da) neurons. One of the fmi-mutant phenotypes was overgrowth of branches in the early stage of dendrite development. In da neurons but not in their adjacent non-neuronal cells, expression of a truncated form (deltaN) that lacks the entire cadherin repeat sequence, rescues flies--at least partially--from this phenotype. Another phenotype is observed at a later stage, when dendritic terminals outgrowing from the contralateral sides meet and then avoid each other. In the fmi mutant, by contrast, those branches overlapped. Overexpression of the deltaN form on the wild-type background phenocopied the overlap phenotype in the mutant, and analysis in heterologous systems supported the possibility that this effect might be because the Fmi-Fmi homophilic interaction is inhibited by deltaN. We propose that a dual molecular function of Fmi play pivotal roles in dendrite morphogenesis. In the initial growing phase, Fmi might function as a receptor for a sofar-unidentified ligand and this hypothetical heterophilic interaction would be responsible for limiting branch elongation. At a later stage, homophilic Fmi-binding at dendro-dendritic interfaces would elicit avoidance between dendritic terminals.