The first germ cells in the hemaphrodite differentiate into sperm. Later in the adult, spermatogenesis is arrested and all subsequent germ cells differentiate into oocytes. This switch is dependent on the post-transcriptional repression of the fem-3 mRNA. In the absence of fem-3, hermaphrodites develop into oocyte-producing females. In contrast, abnormally high levels of fem-3 lead to masculinized germlines. Using genetic and biochemical approaches, we have identified several genes that are necessary for the repression of the fem-3 mRNA. These genes are being characterized in our laboratory.