Sex in Caenorhabditis elegans (XX hermaphrodite, XO male) is determined by the X:A ratio, which is the ratio of X chromosome number to autosomal set number. Recent genetic results with X chromosome duplications have suggested that there may be only a small number of major numerator sites on the X chromosome that contribute to this ratio. Mapping of duplication endpoints delimited a region of less than 300 kb, likely to contain one such element. Cosmid clones from this region were tested for numerator activity by constructing transgenic lines carrying extra copies of each tested cosmid. Most cosmid arrays have no effect on the viability of either XX or XO animals. One cosmid array was found to be viable in XX animals, but lethal and feminizing in XO animals, consistent with it containing a major numerator element. Further experiments defined a region of 12-30 kb with apparent numerator activity, which is designated fox-1, 'Feminizing locus On X'. A cDNA clone hybridizing across part of this region encodes a predicted RNA-binding protein.