The fem-3 sex-determining gene is repressed post-transcriptionally via a regulatory element in its 3' untranslated region (UTR) to achieve the switch from spermatogenesis to oogenesis in the Caenorhabditis elegans hermaphrodite germ line. In this paper, we investigate the fem-3 3' UTR control in somatic tissues using transgenic reporter assays, and we also identify six genes essential for this control. First, we find that a reporter transgene bearing a wild-type fem-3 3' UTR is repressed in somatic tissues, whereas one bearing a mutant fem-3 3' UTR is derepressed. Moreover, control by mutant 3' UTRs is temperature sensitive as predicted from the temperature sensitivity of the fem-3 gain-of-function (gf) mutations. Secondly, we find a fem-3 3' UTR RNA-binding activity in somatic tissues, in addition to the previously reported germ-line-specific binding by FBF. Thirdly, we find that each of six genes, mog-1-mog-6, is required for repression by the fem-3 3' UTR. Therefore, the mog genes not only affect the sperm/oocyte switch in the germ line, but also function in somatic tissues. We suggest that the mog genes may encode components of a ubiquitous machinery that is used for fem-3 3' UTR-mediated repression and the sperm/oocyte switch.