We analyzed the sociogenetic organization of the ant (Leptothorax acervorum) from Nūrnberger Reichswald in Southern Germany. According to relatedness estimates from allozyme analyses, virgin female sexuals produced in polygynous colonies were on average full sisters, whereas workers in a pooled sample of polygynous colonies were significantly less closely related. Rather than attributing this to reproductive hierarchies among nest mate queens, we show how this phenomenon could result from seasonal fluctuations of colony composition and a decline of the production of female sexuals in polygynous colonies. We suggest that by queen adoption and emigration or budding, colonies easily switch from monogyny to polygyny and vice versa. Due to the long developmental time of sexual larvae, colonies that have become polygynous only recently will still produce the female sexual progeny of a single queen. In older polygynous nests, fewer and fewer female sexuals are produced, but colonies may fragment into monogynous buds in which the production of female sexuals may begin again. Relatedness estimates, dissection results, and field observations support this suggestion. This pattern of cyclical monogyny and polygyny keeps nest mate relatedness high and probably facilitates colony founding in boreal habitats. Preliminary data suggest that the pattern of the production of sexuals in colonies of L. acervorum fits the expectations of sex allocation theory.