The oskar (osk) gene encodes a determinant of posterior identity in Drosophila, and the localization of osk RNA to the pole plasm at the posterior pole of the oocyte is essential for development of the embryo. The mechanisms by which osk RNA is localized are unknown.To study the mechanisms underlying localization of osk RNA, we have injected fluorescently labelled RNA into oocytes at stages 9, 10 and 11. Injected osk RNA localizes to the pole plasm, reproducing localization of the endogenous RNA. In oocytes at stages 10 and 11, the long-range movement of injected osk RNA is promoted by a vigorous, microtubule-dependent cytoplasmic flow, or ooplasmic streaming. Treatment with colchicine, a microtubule-destabilizing drug, inhibits ooplasmic streaming and prevents localization of the RNA from an injection site distal to the posterior pole. If the RNA is injected close to the posterior pole, however, it localizes even in the presence of colchicine. Similarly, in small oocytes, such as stage 9 oocytes, localization of injected osk RNA is insensitive to colchicine.These results reveal that microtubule-dependent cytoplasmic flows could contribute to the long-range transport of osk RNA, whereas microtubule-independent processes could mediate short-range transport. These results also highlight the role of the osk RNA anchor in the localization process.