Foraminiferal assemblage found in Upper Eocene deposits from Siemieil (Eastern Poland) includes over 70 species. This assemblage lived in cold shelf waters 80-100 m deep. The foraminifera-bearing deposits may be correlated with lower part of marls of Kiev stage from Ukraine, representing the lower horizon of the Upper Eocene ancIJor the Middle-Upper Eocene junction beds. Foraminiferal assemblage from Siemieil beds is entir,ely different from that known from the stratotype of the Bartonian. Marine transgression responsible for deposition of Siemieil beds presumably reached the area of Poland from the East, utilizing· old tectonic frame: Dnepr-Donetz aulacogen and its extensions. This is confirmed by a marked similarity of foraminiferal assemblages as well as composition of heavy minerals present in deposits of Siemieil beds.