glp-1 encodes a Notch family transmembrane receptor required for mitotic proliferation of the C. elegans germline. Mutations in glp-1 lead to reduced germline proliferation, with strong loss-of-function mutations rendering hermaphrodites sterile. ego-3 was identified in a screen for genetic enhancers of the mild proliferation defect in glp-1(bn18ts) mutants raised at 20°C. The screen was designed to identify mutations that decrease GLP-1/NOTCH signaling in the germline. The best studied allele of ego-3, ego-3(om40), has a complex recessive phenotype that includes reduced and delayed larval germline proliferation, slow development characterized by extended larval period, proximal germline proliferation in adults, delayed and abnormal gamete formation, and a severe mobility defect that improves to nearly wildtype mobility in adults. Another allele obtained by non-complementation, ego-3(om118), has a recessive embryonic lethal phenotype. To identify ego-3(om40), we first carried out three-factor and SNP mapping, which placed ego-3 on chromosome VR, to the left of unc-61 and close to daf-21, in a region containing 14 known protein-coding genes. We next conducted whole genome sequencing (WGS) on homozygous ego-3(om40) mutants. WGS identified one sequence variant within the protein coding region of one gene in the mapped region, daf-21, which encodes the C. elegans ortholog of the molecular chaperone HSP90. The sequence variant is a T>A transition causing an I>N non-conservative missense mutation. Sanger sequencing confirmed this mutation; subsequently, we identified the ego-3(om118) mutation, a 20 bp non-tandem duplication within the same exon containing the ego-3(om40) mutation. ego-3(om118) is predicted to cause a frameshift and introduce a premature termination codon. ego-3(om40) fails to complement daf-21(ok1333), a putative null allele carrying a large deletion. daf-21 RNAi partially phenocopies ego-3 in a wildtype background, causing sterility, embryonic lethality, reduced germline proliferation in the larva, and, in some adults, proximal germline proliferation. Thus, by various genetic and molecular criteria, we have confirmed the identity of ego-3 as daf-21. While HSP90 has long been known to participate in a variety of oncogenic, proliferative signaling pathways in metazoans, our results provide the first evidence that HSP90 plays a role in GLP-1/NOTCH signaling and in mitotic proliferation in the C. elegans germline.