Sex is determined in Caenorhabditis elegans by an X-chromosome-counting mechanism that reliably distinguishes the twofold difference in X-chromosome dose between males (1X) and hermaphrodites (2X)(1,2). This small quantitative difference is translated into the ’on/off’ response of the target gene, xol-1, a switch that specifies the male fate when active and the hermaphrodite fate when inactive(3), Specific regions of X contain counted signal elements whose combined dose sets the activity of xol-1 (ref. 4). Here we ascribe the dose effects of one region to a discrete, protein-encoding gene, fox-1. We demonstrate that the dose-sensitive signal elements on chromosome X control xol-1 through two different molecular mechanisms. One involves the transcriptional repression of xol-1 in XX animals. The other uses the putative RNA-binding protein encoded by fox-1 to reduce the level of xol-1 protein, These two mechanisms of repression act together to ensure the fidelity of the X-chromosome counting process.