The middle flow basin of the Ipeľ river belongs to the valuable territories from the point of view of trans-boundary otter populations (Lutra lutra). Results of the common Slovak-Hungarian research show, that otter permanently occurs on all larger river flows in the observed territory. Regarding to changing discharges of the majority of flows, it comes to changes in the dimension and transfer of its domestic regions. The otter domestic regions on the Slovak side are prevailingly linear and bound mainly to the river flows. On the Hungarian side the occurrence of the otter is predominantly localized to the wetlands. From the negative factors influencing the otter population in the observed territory there are mainly unstableness of the river flow discharge, drying up of some flows in summer period, destruction eventually total absence of littoral vegetation, but also the lack of food (mainly fishes) on some flows. As regards the anthropic factors the interferences to the otter habitats, direct hunting and influence of pie-dogs are prevailing. The Ipeľ river middle flow basin is of great significance also for the other international activities focused to the research and protection of the otter.