Technical Abstract: Fruit fly pest species have been successfully controlled and managed via the Sterile Insect Technique (SIT), a control strategy that uses infertile matings of sterile males to wild females to reduce pest populations. Biological efficiency in the field is higher if only sterile males are released in SIT programs and production costs are also reduced. Sexing strains developed in the Mediterranean fruit fly Ceratitis capitata (medfly) through classical genetics are immensely beneficial to medfly SIT programs but exhibit reduced fertility and fitness, and transfer to other tephritid species is difficult. Transgenic approaches, which can possibly overcome the problems of classical genetic sexing strains (GSSs), have resulted in transgenic sexing strains (TSSs) based on dominant lethality mediated, however, mostly at pupal stages. Here we present a transgene-based female-specific embryonic lethality system for embryonic sexing in medfly. The system utilizes the sex-specifically spliced transformer intron to restrict ectopic mRNA translation of the pro-apoptotic gene hidAla5 to only females when driven by a tetracycline-repressible transactivator gene tTA regulated by promoters/enhancers of cellularization genes. We have tested this system in the medfly C. capitata and, despite of observed position effects on the sex-specific splicing, we could establish the system as desired. After satisfactory performance in large-scale tests, TSSs from such a system will offer cost-effective sexing of pest insect strains once introduced into SIT programs and are straight forward to develop for other insect pest and vector species.