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Summary - In the woodlouse Armadillidium vulgare, the basis of sex determination is under the control of heterochromosomes (ZZ males and WZ females). However, in most populations, sex is determined by 2 maternally-transmitted parasitic sex factors (PSF). One is a Wolbachia-like bacterium (F), the other (f) is probably a sequence of F DNA unstably integrated in the A vulgare genome. Both PSF are feminizing factors, which transform genetic males into functional neo-females (ZZ+F or ZZ+f). The propagation of these PSF was investigated by introducing neo-females into artificial populations initially formed of genetic females. In the majority of these populations, F bacteria were unable to spread, while f seemed to have a higher invasive power. These results contrast with theoretical data: owing to the high transmission rate of PSF, their frequency should increase in populations where they have appeared. Based on the observation that PSF in populations have a behaviour similar to that of genes with selective advantages, a new approach was proposed to describe the spreading of PSF in limited-sized populations. This model showed that even a high transmission rate and in the absence of nuclear genes conferring resistance in A vulgare, PSF can be lost from populations. The equilibrium probability of their disappearance depends on both the number of neo-females infesting the population and the transmission rate of PSF. These theoretical data, combined with the presence of genes repressing the feminizing effects of PSF, may explain part of the variability observed in sex determining systems and sex ratios of natural populations.