The ultrastructural features of oospore wall ornamentation in endemic and critically endangered Balkan Charophytes were examined using scanning electron microscopy (SEM). The present study shows for the first time, in this amount of detail, the interspecific variations in the oospore of endemic and rare Charophytes in the genus Chara from the Balkan Peninsula, which will assist in the correct determination and taxonomy of this genus. Oospore features are presented in detail as the background for some additional tips, including the length and width of the oospore, the fossa width, the number of striae and the length to width ratio. Based on oospore ultrastructure, the taxonomic relationships between the endemic Chara conniventi-fragilis and C. connivens, C. visianii and C. intermedia species are discussed.