Lacustrine sediments at the Łuków site bear a record of the Ferdynandovian interglacial, correlated with Marine Isotope Stage (MIS) 13–15, including two warm periods of interglacial rank (climatostratigraphic units Ferdynandovian 1 and 2) separated by cooling/glaciation (Ferdynandovian 1/2). On the basis of plant macroremains analysis, the type of local vegetation in the lake and its surroundings as well as changes in climate, trophic conditions and water level were reconstructed in detail. Ferdynandovian 1 was a time of development of tall sedge swamps. The presence of Najas marina and N. minor also suggests high levels of eutrophication, particularly in the younger part of the climatic optimum. The occurrence of Zannichellia palustris indicates habitats of variable water level and high salt content. In the terminocratic phase of Ferdynandovian 1, the communities showed the reoccurrence of Betula nana, B. humilis and Larix sp., the disappearance of thermophilous trees, and the intensification of succession processes linked to climate cooling. In the cool Ferdynandovian 1/2, Betula nana and Cenococcum geophilum increased their frequencies, most likely due to enhanced supply of mineral matter to the basin. During Ferdynandovian 2, the next climate warming of interglacial rank, communities of aquatic vegetation with the highest share of thermophilous taxa included the extinct Aldrowanda borysthenica, Brasenia borysthenica, and Scirpus atroviroides, as well as Cyperus glomeratus, a species not presently found in the flora of Poland. Another cooling in the Sanian 2 (Elsterian 2) glaciation is indicated by the development of peat communities, with numerous Carex sp., Menyanthes trifoliata, Eriophorum vaginatum, and Andromeda polifolia, accompanied by the extinct Carex paucifloroides, Caulinia macrosperma, and Potamogeton praemaackianus. The provided description of the Ferdynandovian succession includes taxa which are extinct, not found in the Polish flora nowadays, or characteristic of a climatic context. The identified species included Brasenia borysthenica, Aldrovanda borysthenica, Caulinia macrosperma, Potamogeton praemaackianus, Scirpus atroviroides, Cyperus glomeratus, Eleocharis praemaximoviczii, Nymphaea cinerea, and Ranunculus gailensis. It has not yet been resolved whether the flora of Łuków includes Euryale sp., a particularly important taxon.