Cyclotella iris Brun & Herib. and its varieties have been studied by LM and SEM. They were represented in Neogene continental deposits of the Sofia Basin, South Bulgaria with rich abundance. New data about the fine structure of the Cyclotella iris var. integra Perag. & Herib. were obtained and this made it possible to add the morphological characteristics of the Cyclotella iris group. In the informal subdivision of the genus Cyclotella this species and its varieties would fall within separate subgroup.