In 1977 the Babia Gora National Park (the BPN) was included by UNESCO in the list of the international network of the Biosphere Reserves that comprise the world's protected ecosystems.The investigation, which is being carried out in the area of the BPN, aims at finding and recording species of the zooplankton as well as the Paramecium aurelia species complex occurring in the lakes of Mt Babia Gora. The investigation was carried out in Mokry Stawek, one of the largest lakes in the area. This is a preliminary report. It reveals a significant acidity of the lake and that so far none of the examined species of the Paramecium aurelia complex have been found. However, some predominant zooplanktonic organisms were registered.