Sex is determined by non‐Mendelian genetic elements overriding the sex factors carried by the heterochromosomes in some species of terrestrial isopods. A bacterium Wolbachia and a non‐bacterial feminizing factor (f) can both force chromosomal males of Armadillidium vulgare to become phenotypic functional females. The f factor is believed to be a genetic element derived from the Wolbachia genome that becomes inserted into the host nuclear genome. The feminizing factors can be considered to be selfish genetic elements because they bias their (...) host's sex ratio to increase their own transmission. New sex‐determining genes are selected (genes resisting the feminizing effects, or the transmission of feminizing elements) as a consequence of the conflict between these elements and the rest of the host's genome. These events drive the sex‐determining mechanisms to evolve, and may explain the polymorphism of sex factors and the poor differentiation of the heterochromosomes in isopods. (shrink)