Aedeagus of the type species of seven subgenera in the genus Cassida Linnaeus, 1758 have been studied and figured. Twenty aedeagal structures of these species of were evaluated in detail under both stereo microscope and Scanning Electron Microscope (SEM).Contrary of popular acceptance, it is demonstrated that the characteristics of the aedeagal structure, which can be obtained with SEM studies especially, can be diagnostic at almost all taxonomic levels The characters; however, will have different values for different taxonomic categories. As a result of this study, seven diverse aedeagal characters are found generally constant within species, but distinct and useful in comparison with other species [as 1. characteristics of apex in dorsal view (D2), 2. general shape in lateral view (L7), 3. the ratio of the distance between posterior margin of the dorsal plate and the apex to the entire length of the median tube in dorsal view (D11), 4. the ratio of the width of apical part to the width of basal part of median lobe in lateral view (L4), 5. the ratio of the width of apex to the width of apical part in dorsal view (D3), 6. surface structure of the apical part in front of orifice including apex in dorsal view (D5), 7. surface structure behind orifice in dorsal view (D6)]. Also, as an important output of the study, it is observed that the type species and therefore the subgenera can be divided chiefly into 3 diverse group based on aedeagal structures: C. seraphina in C.(Alledoya), C. nebusa in C. (s. str.) and C. hemisphaerica in C.(Mionychella) as Group I; C. nobilis in C.(Cassidulella) as Group II; C. viridis in C .(Odontionycha), C. brevisin C.(Onychocassis)and C. murrea in C. (Pseudocassida) as Group III. The new grouping is discussed with the grouping according to host plants and with the grouping in previous stereo microscopic works due to there is no available SEM studies. These previous groupings do not seem to be compatible with the results obtained with this study. Based on the ultrastructure of aedeagus, the genus Cassida Linnaeus, 1758 is a polyphyletic group, not monophyletic.In addition, as a side outcome of the work, Cassida hemispherica Herbst, 1799 is reported for the first time with a verified locality record from Turkey. Moreover, the habitus of adults of the species examined and a diagnostic key to species examined based on the external and aedeagal morphological characters of adults is also provided.