The author was able to observe melosira ambigua in large numbers in the plankton of the Irtysh River, Lake Zaysan, several lakes in Omskaya Oblast, the Verkh-Isetskiy Pond in the Urals (from the materials of S.N. Ulomskiy), the Dunay River (from the materials of Ya. V. Roll) and so on. It was established that among the vast majority of M. ambigua cells with the typical, more or less deep annular groove, cells were encountered in which the groove did have an acute-angled form resembling that of M. italica. Further study showed that these 'italica'-shaped cells were juvenile. The research by the author aids in establishing the systematic position of Melosira ambigua.