PGL-1 is a constitutive protein component of C. elegans germ granules, also known as P granules. Maternally supplied PGL-1 is essential for germline development but only at elevated temperature, raising the possibility that redundant factors provide sufficient function at lower temperatures. We have identified two PGL-1-related proteins, PGL-2 and PGL-3, by sequence analysis of the C. elegans genome and by a yeast two-hybrid screen for proteins that interact with PGL-1. PGL-3 is associated with P granules at all stages of development, while PGL-2 is associated with P granules only during postembryonic development. All three PGL proteins interact with each other in vitro. Furthermore, PGL-1 and PGL-3 are co-immunoprecipitated from embryo extracts, indicating that they are indeed in the same protein complex in vivo. Nevertheless, each PGL protein localizes to P granules independently of the other two. pgl-2 or pgl-3 single-mutant worms do not show obvious defects in germline development. However, pgl-1; pgl-3 (but not pgl-2; pgl-1) double-mutant hermaphrodites and males show significantly enhanced sterility at all temperatures, compared to pgl-1 alone. Mutant hermaphrodites show defects in germline proliferation and in production of healthy gametes and viable embryos. Our findings demonstrate that both PGL-2 and PGL-3 are components of P granules, both interact with PGL-1, and at least PGL-3 functions redundantly with PGL-1 to ensure fertility in both sexes of C. elegans.