It is accepted that male genitalia are not diagnostic and spermathecae are partly diagnostic within the genus Cassida Linnaeus, 1758 (Coleoptera: Chrysomelidae: Cassidinae). However, studies on genitalia have been based on only stereo microscope up to now. Ultra-structures of genitalia have never been studied except for a few works. The aim of this study was to determine whether the ultrastructural works are efficient or not. In the present work, male genitalia from two specimens of Cassida prasina Illiger, 1798 collected from Duzce and Karabuk provinces in 2001 and 2003, which have been examined by electron microscope for the first time. New diagnostic characters were obtained and it revealed that it was diagnostic from the previously worked species in others subgenus. Photos of aedeagus in both SEM and stereo microscope are also given in the text.