The migration of border cells during Drosophila melanogaster oogenesis is a simple and powerful system for studying invasive cell migration in vivo. Border cells are somatic cells that delaminate from the follicular epithelium of an egg chamber and invade the germ line cluster. They migrate between the nurse cells to reach the oocyte, using DE-cadherin for adhesion to the substratum. Border cells take approximately 6 h to migrate a distance of 100 microm. The migration is guided by EGFR (epidermal growth factor receptor) and PVR (platelet-derived growth factor (PDGF)/vascular endothelial growth factor (VEGF) receptor). Here, we show that a single long cellular extension (LCE), several cell diameters in length, is formed at the initiation of migration. The LCE may function as a 'pathfinder' in response to guidance cues. LCE growth requires directional guidance signals and specific adhesion to the substratum. Interference with actin-myosin interactions allows continued LCE growth while preventing translocation of the cell bodies. We discuss similarities between LCEs and axons and the use of LCE-like structures as a general mechanism for initiating invasive migration in vivo.