Abstract: The host range of Marssonina populi and M. castagnei, and the relative susceptibility of various poplars to these fungi, were determined through laboratory inoculation tests and a field study. Nine species or hybrids of Populus in sect. Aigeiros and Tacamahaca were found susceptible to M. populi, and five species or hybrids in section Populus (Leuce) proved to be susceptible to M. castagnei. The life cycle of M. populi was also studied. M. castagnei was recorded for the first time in Finland.