DAZ (Deleted in Azoospermia), the putative azoospermia factor gene in human, encodes a ribonucleoprotein-type RNA-binding protein required for spermatogenesis. A Drosophila homologue of DAZ, called boule, is also essential for spermatogenesis. A mouse homologue, Dazla, is implicated in both spermatogenesis and oogenesis. Here, we report the identification and characterization of daz-1, the single DAZ homologue in the nematode Caenorhabditis elegans. Loss of daz-1 function caused sterility in hermaphrodites, by blocking oogenesis at the pachytene stage of meiosis I. Epistasis analysis suggested that this gene executes its function succeeding gld-1, which governs the early pachytene stage in the oogenic pathway. Spermatogenesis did not appear to be affected in daz-1 hermaphrodites. Males defective in daz-1 produced sperm fully competent in fertilization. Analysis employing sex-determination mutants indicated that the daz-1 function was required for meiosis of female germline regardless of the sex of the soma. Transcription of daz-1 was restricted to the germline, starting prior to the onset of meiosis and was most conspicuous in cells undergoing oogenesis. Thus, daz-1 in C. elegans is an essential factor for female meiosis but, unlike other DAZ family members so far reported, it is dispensable for male meiosis.