Hydrocharitaceae is a cosmopolitan aquatic group that comprises 17 genera with 127 species. The individuals are herbs fixed or free, floating, submerged or emerged living in fresh, brackish or salty water. In Brazil, it is represented by six genera and ca. 15 species. Aiming to know the real diversity of the family in Rio de Janeiro state, a study of herbaria collection and an extensive field work were made. Six native and two introduced species were registered. Apalanthe granatensis, presents bisexual flowers and was found in dams, rivers and freshwater ponds. Egeria densa, is similar to A. granatensis, and can be distinguished by the unisexual flowers. Halophila decipiens, occurs in the Guanabara Bay beaches, it is characterized by rhizomatous stem and elliptical leaves. Limnobium laevigatum. Heine is a free floating herb found in ponds and emerged in marshes, it has floating leaves with abaxial aerenchymatous thickening and/or emerged leaves without thickening. Najas arguta var. arguta exhibits spineless stem, leaves concentrated at the apex of the branches and fruit with fusiform seed, and Najas marina has thorns on the stem and midrib, and elliptical seed. Both are found in coastal lakes. Elodea canadensis and Vallisneria sp. were found under cultivation.
Southeast Brazilian flora; Atlantic Forest; Najadaceae; aquatic plants