oskar ( osk) mRNA encodes the Drosophila posterior determinant and its correct localization in the oocyte is essential for embryonic development. oskar mRNA is synthesized in the nurse cells during early oogenesis and transported and anchored to the posterior pole of the oocyte by a mechanism that is still unclear. Understanding the mechanism of osk mRNA movement during oogenesis requires the identification of the key determinants of its transport at the molecular level. I propose to develop an assay based on the analysis of the movement of osk RNA particles at high resolution by time-lapse confocal microscopy. osk mRNA transport will be visualized in living egg chambers after injection of fluorescent labeled RNA, as well as in vitro, in a reconstituted system. These assays, combined with biochemistry and genetics will be used to identify: i) nuclear components responsible of osk mRNA exit from the nurse cell nuclei; ii) osk RNA interacting proteins which could define the localization of osk in the oocyte cytoplasm during the different steps of Drosophhila oogenesis; iii) cellular proteins which could link osk mRNA to the cytoskeletal network: iv) cytoskeletal motors required for ask active transport through the egg chamber and v) the cis-acting signals present in osk mRNA involved in its transport.