The occurrence of several specimens of Heterostegina reticulata Rütimeyer is reported for the first time in the latest Middle Eocene limestones at Mossano (Berici Mts., Veneto, Italy). The species was considered exclusive to the Upper Eocene; it was found here in association with Nummulites biedai, N. lyelli, N. maximus and N. ptukhiani, markers of the upper part of the Middle Eocene and therefore has a wider range than previously considered. Moreover biometrical analysis of three populations of Heterostegina reticulata from different stratigraphic levels (Middle and Upper Eocene) of the same locality showed a clear evolutionary trend towards increasing size of the embryonal apparatus and early development of the secondary chambers.