In screens for genetic modifiers of lin-35/Rb, the C. elegans retinoblastoma protein (Rb) homolog, we have identified a mutation in xnp-1. Mutations in xnp-1, including a presumed null allele, are viable and, in general, appear indistinguishable from the wild type. In contrast, xnp-1 lin-35 double mutants are typically sterile and exhibit severe defects in gonadal development. Analyses of the abnormal gonads indicate a defect in the lineages that generate cells of the sheath and spermatheca. xnp-1 encodes the C. elegans homolog of ATR-X, a human disease gene associated with severe forms of mental retardation and urogenital developmental defects. xnp-1/ATR-X is a member of the Swi2/Snf2 family of ATP-dependent DEAD/DEAH box helicases, which function in nucleosome remodeling and transcriptional regulation. Expression of an xnp-1 Colon, two colons GFP promoter fusion is detected throughout C. elegans development in several cell types including neurons and cells of the somatic gonad. Our findings demonstrate a new biological role for Rb family members in somatic gonad development and implicate lin-35 in the execution of multiple cell fates in C. elegans. In addition, our results suggest a possible conserved function for xnp-1/ATR-X in gonadal development across species.