ABSTRACT The C. elegans germ line is generated by extensive proliferation of the two germ-line progenitor cells present in newly hatched larvae. We describe genetic and phenotypic characterization of glp-4, a locus whose product is required for normal proliferation of the germ line. glp-4(bn2ts) mutant worms raised at the restrictive temperature contain approximately 12 germ nuclei, in contrast to the 700-1000 present in wild-type adults. The few germ cells present in sterile glp-4 adults appear to be arrested at prophase of the mitotic cell cycle. This cell-cycle disruption prevents the germ cells from entering meiosis and differentiating into gametes. Shifting sterile glp-4 worms to the permissive temperature enables their germ cells to undergo extensive proliferation and form gametes, demonstrating that the bn2-induced cell-cycle arrest is reversible and that proliferation and differentiation of germ cells can be uncoupled from development of the somatic gonad. The glp-4(bn2ts) mutation can be used to generate large populations of worms that are severely depleted in germ cells, facilitating determination of whether any gene of interest is expressed in the germ line or soma or both.