For the first time sexual reproduction and mating system of the freshwater diatom Ulnaria acus (Bacillariophyta) was described. Mating system of the species includes, apparently, only heterothallic way of reproduction: during the three-year observation period no male nor female clones did show the ability to reproduce gomotallically. Cis-anisogamous type of sexual reproduction in many details resembles reproduction of closely related species, Ulnaria. ulna, Tabularia fasciculata, Tabularia tabulata.