Cellularization of the Drosophila embryo is a specialized form of cytokinesis that couples membrane growth with the formation of a polarized epithelium. We have identified a gene essential for polarized growth of the plasma membrane during cellularization. In slam mutant embryos, the furrow canal is disorganized, and polarized insertion of transmembrane proteins is disrupted. slam shows a striking developmental induction during the slow phase of cellularization, and Slam protein localizes to the furrow canal and the basal junction. Slam colocalizes with the junctional proteins Arm/beta-catenin, the PDZ domain-containing protein Dlt, and Myosin and is also required for their proper membrane localization. Our results suggest that developmental induction of Slam organizes the polarized growth of membrane via the recruitment of membrane-targeting proteins at adherens junctions.