Lacustrine deposits from Ortel Królewski II (Eastern Poland) represent the Holsteinian Interglacial (MIS 11c). They are characterized by an extremely rich occurrence of ostracod and mollusc fauna. Collected samples represent pre-optimal part of the Holsteinian Interglacial corresponding to Picea–Alnus, Taxus and Pinus–Larix zones. Based on ostracod assemblage analysis a depth of the paleolake, the energy of the environment and the average January and July air temperature were reconstructed. Ostracods from Ortel Królewski II indicate a lake with possible periodic overflow sur- rounded by periodically flooded grasslands, which existed in the study area during the pre-optimal part of Holsteinian Interglacial.

Within a small (2.5 x 2.5 km) test field, located in the eastern part of the Pomeranian Bay, the geological structure was investigated in detail using seismoacoustic profiling and coring. In the cores, mineral grain size and micro- and macrofauna were analysed. Basing on seismoacoustic records, three main seismostratigraphic units were distinguished. The lowest unit is built of sand and muddy sand. Higher up lies the unit of sandy mud, in some places - of clay. Fauna assemblages indicate cold climate conditions, and clearly point to an existence of lacustrine reservoir of oligotrophic character. Analysis of cores shows that sedimentation began in bog conditions. The lakes in the area existed till the Atlantic period, when due to sea transgression they became filled with sandy sediments. The third unit is built mainly of fine sand, locally medium sand, even with addition of gravel. Basing on the presence of marine fauna, this unit is classified as marine sand.

The assemblage of lacustrine ostracods found in the Eemian Interglacial sediments at Kruklanki (Masurian Lake District, northeastern Poland) contains 18 species belonging to 13 genera. The most dominant species are Candona neglecta Sars, 1887, Limnocytherina sanctipatricii (Brady et Robertson, 1869), Limnocythere inopinala (Baird, 1843) and Candona candida (O.F. Muller, 1776). Cyclocypris serena (Koch, 1838), llyocypris decipiens Masi, 1905, Pseudocandona insculpta (G.W. Muller, 1900) and Leucocythere mirabilis Kaufmann, 1892 are recorded for the first time from the Eemian of Poland; the latter two species are also new for the Eemian lacustrine deposits of Europe. The ecological requirements of the recognised ostracod species as well as their geographic ranges in the Quaternary of Europe are summarised. Based on these data, past habitat type is estimated as a deeper littoral of a lake with reasonably cold, well-oxygenated and calcium-rich waters. The present state of knowledge of the Eemian ostracods from Poland is reviewed and their comparison with the Eemian ostracod assemblages from Europe is briefly given. Comparison of the ostracod fauna! assemblage from Kruklanki with those from other Eemian sites in Poland enables to establish and describe one general type of ostracod assemblages characteristic for lacustrine littoral in this interglacial.