ABSTRACT Receptor tyrosine kinases have been shown to promote cell movement in a variety of systems. The Ras protein, a well-documented downstream effector for receptor tyrosine kinases, may contribute to receptor tyrosine kinase-mediated motility. In the present study, we have examined the role of Ras in the migration of a small subset of follicle cells, known as the border cells, during Drosophila oogenesis. A dominant-negative Ras protein inhibited cell migration when expressed specifically in border cells during the period when these cells normally migrate. When expressed prior to migration, dominant-negative Ras promoted premature initiation of migration. Conversely, expression of constitutively active Ras prior to migration resulted in a significant delay in the initiation step. Fur-thermore, the defect in initiation of border cell migration found in slbo1, a mutation at the locus that encodes Drosophila C/EBP, was largely rescued by reducing Ras activity in border cells prior to migration. Taken together, these observations indicate that Ras activity plays two distinct roles in the border cells: (1) reduction in Ras activity promotes the initiation of that migration process and (2) Ras activity is required during border cell migration. We further examined the possible involvement of two downstream effectors of Ras in border cell migration. Raf activity was dispensable to border cell migration while reduced Ral activity inhibited initiation. We therefore suggest that Ras plays a critical role in the dynamic regulation of border cell migration via a Raf-inde-pendent pathway.