Investigations of the Lower Miocene of the Transylvanian Basin reveal particularly high abundances (>90 % of total foraminifera) of small sized biserial foraminifera (Bolivina/Streptochilus). This biotic event has not been noticed in the Transylvanian Basin so far probably owing to the facies misinterpretation and the small size of the specimens. SEM investigations allow more precise identification of biserial planktonic taxa and more accurate taxonomic interpretations. The high abundance of Bolivina/Streptochilus assemblages provide evidence for paleogeographic connections to the Indo-Pacific area and support new paleoenvironmental interpretations at the transition from the Early to Middle Miocene in relation to the paleoceanographic events. Both planktonic foraminifera and calcareous nannoplankton suggest a late Burdigalian age. A new Bolivina/Streptochilus Abundance Biozone is proposed just below the Early/Middle Miocene boundary.