Receptor for Activated CKinase 1 (RACK1) is a cytoplasmic molecular scaffolding protein. Many diverse protein-binding partners involved in key signaling pathways are reported to bind to RACK1, suggesting a role for RACK1 in signal integration. However, because loss-of-function phenotypes for RACK1 in an intact organism have not yet been reported, our current understanding of RACK1 is limited. Using Drosophila melanogaster, we show that RACK1 is expressed at all developmental stages and in many tissues, with specific enrichment in the ovary. By characterizing an allelic series of RACK1 mutants, we demonstrate that RACK1 is essential at multiple steps of Drosophila development, particularly in oogenesis, where somatic RACK1 is required for proper germ-line function. Developmental Dynamics 236:2207–2215, 2007. © 2007 Wiley-Liss, Inc.