In Drosophila, the posterior determinant nanos is required for embryonic patterning and for primordial germ cell (PGC) development. We have identified three genes in Caenorhabditis elegans that contain a putative zinc-binding domain similar to the one found in nanos, and show that two of these genes function during PGC development. Like Drosophila nanos, C. elegans nos-1 and nos-2 are not generally required for PGC fate specification, but instead regulate specific aspects of PGC development. nos-2 is expressed in PGCs around the time of gastrulation from a maternal RNA associated with P granules, and is required for the efficient incorporation of PGCs into the somatic gonad. nos-1 is expressed in PGCs after gastrulation, and is required redundantly with nos-2 to prevent PGCs from dividing in starved animals and to maintain germ cell viability during larval development. In the absence of nos-1 and nos-2, germ cells cease proliferation at the end of the second larval stage, and die in a manner that is partially dependent on the apoptosis gene ced-4. Our results also indicate that putative RNA-binding proteins related to Drosophila Pumilio are required for the same PGC processes as nos-1 and nos-2. These studies demonstrate that evolutionarily distant organisms utilize conserved factors to regulate early germ cell development and survival, and that these factors include members of the nanos and pumilio gene families.