In the Nearctic ant Leptothorax sp. A, aggressive interactions among wingless intermorphic queens and primarily winged gynomorphic queens lead to the formation of dominance hierarchies, in which the highest-ranking individual is the only egg-layer in a colony. Fighting occurs during two periods of the annual cycle: in late summer, newly adopted young queens are integrated into the colony's hierarchy; after hibernation, fighting resumes and the high aggressiveness of α-queens may now lead to the emigration of β and other middle-ranking queens. The α-position appears to be very stable over successive fighting periods, though an estimate of nestmate relatedness by allozyme electrophoresis (Polyacrylamide gels and cellulose acetate plates) suggests that queen replacement occasionally occurs. The mean relatedness determined for adult workers in a functionally monogynous population of Leptothorax sp. A was 0.54 and their effective number of mothers therefore 1.5. This is lower than expected and found for monogynous colonies. Dominance rank is apparently not correlated with queen morph, weight, and size, but an influence of insemination, age, or previous reproductive experience is likely.