Remnats of cells and branched filaments of carbonate boring (endolithic) algae have been found in the Upper Silurian sedimentary rocks of eastern Poland. The material was obtained from a borehole at Widowo, near Bielsk Podlaski at a core depth of 524–525 m. The age of the sediments at a core depth of 545–585 m was determined as the Upper Silurian (Wenlockian), based on tabulate corals and stromatoporoids. The coral–stromatoporoid assemblage corresponds to the Jaagarahu Stage of the Silurian of Estonia1,2. It includes tabulate and heliolitoid corals Coenites juniperinus Eichwald, Palaeofavosites collatatus Sokolov, P. frivolus Klaaman, P. tersus Klaaman and Heliolites decipiens M'Coy3, and stromatoporoids Densastroma pexisum Yavorsky and Stromatopora impexa Nestor (Kazmierczak, unpublished).