The germ cell precursors of Drosophila (pole cells) are specified by maternally supplied germ plasm localized to the posterior pole of the egg. One component of the germ plasm, germ cell-less (gcl) mRNA, encodes a novel protein which specifically localizes to the nuclear envelope of the pole cell nuclei. In addition to its maternal expression, gcl is zygotically expressed through embryonic development. In this report, we have characterized a null allele of germ cell-less to determine its absolute requirement during development. We have found that gcl activity is required only for the establishment of the germ cell lineage. Most embryos lacking maternal gcl activity fail to establish a germline. No other developmental defects were detected. Examination of germline development in these mutant embryos revealed that gcl activity is required for proper pole bud formation, pole cell formation, and pole cell survival. Using this null mutant we have also assayed the activity of forms of Gcl protein with altered subcellular distribution and found that localization to the nuclear envelope is crucial for promoting pole cell formation, but not necessary to initiate and form proper pole buds. These results indicate that gcl acts in at least two different ways during the establishment of the germ cell lineage.