With a focus on Sex-lethal (Sxl), the master regulator of Drosophila somatic sex determination, we compare the sex determination mechanism that operates in the germline with that in the soma. In both cell types, Sxl is functional in females (2X2A) and nonfunctional in males (1X2A). Somatic cell sex is determined initially by a dose effect of X:A numerator genes on Sxl transcription. Once initiated, the active state of SXL is maintained by a positive autoregulatory feedback loop in which Sxl protein insures its continued synthesis by binding to Sxl pre-mRNA and thereby imposing the productive (female) splicing mode. The gene splicing-necessary factor (snf), which encodes a component of U1 and U2 snRNPs, participates in this RNA splicing control. Here we show that an increase in the dose of snf+ can trigger the female Sxl RNA splicing mode in male germ cells and can feminize triploid intersex (2X3A) germ cells. These snf+ dose effects are as dramatic as those of X:A numerator genes on Sxl in the soma and qualify snf as a numerator element of the X:A signal for Sxl in the germline. We also show that female-specific regulation of Sxl in the germline involves a positive autoregulatory feedback loop on RNA splicing, as it does in the soma. Neither a phenotypically female gonadal soma nor a female dose of X chromosomes in the germline is essential for the operation of this feedback loop, although a female X-chromosome dose in the germline may facilitate it. Engagement of the Sxl splicing feedback loop in somatic cells invariably imposes female development. In contrast, engagement of the Sxl feedback loop in male germ cells does not invariably disrupt spermatogenesis; nevertheless, it is premature to conclude that Sxl is not a switch gene in germ cells for at least some sex-specific aspects of their differentiation. Ironically, the testis may be an excellent organ in which to study the interactions among regulatory genes such as Sxl, snf, ovo and otu which control female-specific processes in the ovary.