This document summarizes an experiment testing the effects of single gene mutations on density-dependent sex differentiation in the fern Ceratopteris richardii. 11 mutant strains and a wild type strain were grown under controlled conditions and the number and sex of gametophytes were counted. Most mutant strains showed similar trends of increasing male percentage with density as the wild type. However, two strains, stl2 and rum1, showed significant differences in male percentage but normal growth, suggesting an altered response to the sex pheromone for those strains. The results provide insight into how genetic mutations can influence sex determination mediated by population density cues.