Assigning offspring to parents is important for understanding the evolution of reproductive conflicts and cooperation, particularly in the model systems represented by social insects. Molecular genetic markers are often used to exclude, and occasionally used to assign, candidate parents. However, their use in social insects has been unsatisfactory so far because candidate mothers are often highly related and candidate fathers are unknown. Here, we show that microsatellite loci can be scored from each mother's stored sperm permitting effective maternity assignment. The theoretical power of this method is huge, and we demonstrate its practical utilization in this large-scale study of the wasp, Polistes annularis. All 219 genotyped daughters were either assigned to a unique mother or shown to be the progeny of an uncollected dead mother. The data reveal an unexpectedly high number of changes in reproductive dominance. Maternity assignments using this method should help solve many difficult questions in social evolution.