The focus of this work is to examine the genetic structure of newly colonized populations of the mycophagous beetle Phalacrus substriatus. The newly colonized populations are characterized by small founding groups, often consisting only of females. Because an all-female colonizing propagule can successfully found a new population, females must be mated prior to dispersal. Furthermore, because of limited dispersal, founders come from only a limited set of populations, thereby increasing their probability of common origin (φ). The small founding groups, high probability of common origin and mating patterns of female beetles generate a strong kin-structuring in P. substriatus as evidenced by the high average relatedness among first generation offspring (r = 0.190, weighted average over populations). The effective number of matings per female (Me) was estimated to be around 1.5. This yields an estimate of the effective size of the newly colonized populations of 5.2. The probability of common origin was also high (0.80). The results are discussed in the light of how founding events may affect the structuring of genetic variation within and between local populations.