How is sex determined? In the animal kingdom, there are diverse sets of mechanisms for determining organismal sex, with the predominant ones being chromosomally based, either a dominant-acting sex chromosome or the ratio of the number of X chromosome to autosomes, which lead to oocyte-producing females and sperm-producing males. The resulting germline sexual phenotype is often the logical consequence of somatic sex determination. In this respect however, the Caenorhabditis elegans hermaphrodite is different from mammals and Drosophila. In fact in the C. elegans hermaphrodite germline, male gametes are transiently produced in a female body during larval development. To override chromosomal signals, sex determination of germ cells strongly depends on post-transcriptional regulation. A pivotal role for male gamete production (spermatogenesis) is played by the fem-3 mRNA, which is controlled through FBF and other RNA-binding proteins or splicing factors. Thanks to its powerful genetics, transparent body, small size, and the ability to make sperm and oocytes within one individual, C. elegans represents an excellent system to investigate cellular differentiation and post-transcriptional control.