IN many animal groups, factors required for germline formation are localized in germ plasm(1), a region of the egg cytoplasm, In Drosophila embryos, germ plasm is located in the posterior pole region and is inherited in pole cells, the germline progenitors. Transplantation experiments have demonstrated that germ plasm contains factors that can form germline(2-4), and germ plasm also directs abdomen formation(5). Genetic analysis has shown that a common mechanism directs the localization of the abdomen and germline-forming factors to the posterior pole(6-12). The critical factor for abdomen formation is the nanos (nos) protein (nanos)(13-15). Here we show that nos is also essential for germline formation in Drosophila; pole cells lacking nanos activity fail to migrate into the gonads, and so do not become functional germ cells. In such pole cells, gene expression, which normally initiates within the gonad, begins prematurely during pole cell migration. Premature activation of genes in germline precursors may mean that these cells fail to develop normally. A function for nos protein in Drosophila germline formation is compatible with observations of its association with germ plasm in other animals(16-18).