Genetic caste determination (GCD), the production of heterozygous sterile workers and homozygous reproductive females, is a widespread phenomenon throughout the hybrid range of Pogonomyrmex rugosus and P. barbatus. GCD may result from direct control of egg genotypes by the queen (queen control hypothesis) or from worker identification and culling of reproductive brood (worker control hypothesis). To test these hypotheses, eggs were collected from newly mated queens and from colonies with workers present. Eggs were then genotyped. Preliminary results suggest that queens lay both heterozygous and homozygous eggs, supporting the worker control hypothesis.