A study of seed coat micromorphology, using both light microscopy and scanning electron microscopy, revealed characters of taxonomic significance and utility among the species of Eriocaulaceae of the United States and Canada. Our survey of nine native species of Eriocaulon, five Lachnocaulon, and one Syngonanthus revealed diversity in the shape and size of the cells of the seed coat, the degree and pattern of ornamentation, and appendage shape. In the context of world Eriocaulaceae, the species of the United States and Canada exhibit no morphologies that are unique.