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Most ferns, unlike all seed plants, are homosporous and produce sexually undifferentiated spores. Sex ratio in many homosporous species is environmentally established by the secretion of antheridiogen from female/hermaphrodite gametophytes. Nearby undetermined gametophytes perceive antheridiogen, which induces male development. In the fern Ceratopteris richardii (Ceratopteris), hermaphroditic (her) mutants develop as hermaphrodites even in the presence of antheridiogen. Modern sequencing and genomic tools make the molecular identification of mutants in the 11-Gbp genome of this fern possible. We mapped 2 linked mutants, her7-14 and her7-19, to the same 16-Mbp interval on chromosome 29 of the Ceratopteris genome. An ortholog of the receptor kinase gene BRASSINOSTEROID INSENSITIVE 1 (BRI1) within this interval encoded a deletion mutation in her7-14 and a missense mutation in her7-19. Three other linked her mutants encoded missense mutations in the same gene, which we name HER7. Consistent with a function as a receptor kinase, HER7-GFP fusion protein localized to the plasma membrane and cytoplasm. Analysis of gene expression showed that brassinosteroid biosynthesis was upregulated in hermaphrodites compared with male gametophytes. Our work demonstrates that HER7 is required for sex determination in Ceratopteris and opens avenues for studying the evolution of antheridiogen systems.