Polarization of a neuron begins with the appearance of the first neurite, thus defining the ultimate growth axis. Unlike late occurring polarity events (such as axonal growth), very little is known about this fundamental process. We show here that, in Drosophila melanogaster neurons in vivo, the first membrane deformation occurred 3.6 min after precursor division. Clustering of adhesion complex components (Bazooka (Par-3), cadherin-catenin) marked this place by 2.8 min after division; the upstream phosphatidylinositol 4,5-bisphosphate, by 0.7 min after division; and the furrow components RhoA and Aurora kinase, from the time of cytokinesis. Local DE-cadherin inactivation prevented sprout formation, whereas perturbation of division orientation did not alter polarization from the cytokinesis pole. This is, to our knowledge, the first molecular study of initial neuronal polarization in vivo. The mechanisms of polarization seem to be defined at the precursor stage.