Infection with symbiontic Wolbachia bacteria has been discovered in nine Trichogramma species, parasitoids of lepidopteran eggs. In the field, populations occur that consist of uninfected and infected individuals of the same Trichogramma species. Uninfected females produce sons from unfertilized eggs and daughters from fertilized eggs. Infected females produce daughters from unfertilized and fertilized eggs. Infected males do not occur. Wolbachia is inherited from mother to daughter. Horizontal transmission is extremely rare. Therefore uninfected and infected forms can be considered as two separated populations living on the same host. According to the 'Competitive exclusion principle' this situation cannot occur without special mechanisms. In this paper we investigate whether a mechanism specific for Wolbachia infection can explain coexistence. Wolbachia has a negative effect on total offspring production, but causes a sex ratio of 100% females. Furthermore uninfected females often sibmate, while infected females never sibmate, because they do not have brothers. Sibmating has a negative effect on offspring production of the inbred female offspring. These mechanisms form the basis for a simple continuous model. By calculating invasion criteria we show that this model can explain coexistence of uninfected and infected Trichogramma forms.