Comprehensive lithostratigraphic and biostratigraphic investigation of diatoms, chrysophycean stomatocysts, and mollusc fauna was performed on a borehole C-14, near the village of Katina, from the peripheral zone of the Sofia Neogene Basin. The diatom flora consisted of 302 species, varieties and forms. Principal Component Analysis (PCA) was used to classify the diatom thanatocoenoses. The results of these different analyses presented here indicated three successive stages in the paleoenvironmental development of the Sofia Basin, at the end of the Miocene and the beginning of the Pliocene.