The aim of the bachelor's thesis was to analyze the occurrence of the roundworms in the digestive tractof great cormorant (Phalacrocorax carbo). During the 2015/2016 season when the hunting of great cormorant wasapproved by the Ministry of Environment of the SlovakRepublic was by the competent user of the hunting area several individuals killed (by guns ) and then subjected to dissectionfor the occurrence of the roundworms (designated as sp.). Numbers and results were processed inthe table form, that is found in the results of the thesis. In other parts of the thesis we dealt concretely with the species of great cormorant and with its occurrence, extension, description of the species,food, habitat and prevention. Habitat, nightlife places and the hunting grounds of great cormorant weremonitored and observed in the locations from Banska Bystrica via Zvolen to Ziar nad Hronom.Nightlife places were predominantly the willows or the alders growing individually or in groups past the banksof the river Hron. These trees were completely or partially dried due to the effectsof great cormorant excretions. The biggest nightlife place in this locality is located in the Zvolen districtin the cadastral area of Budca behind the petrol station Slovnaft. We noticed smaller nightlife placesin the Ziar nad Hronom district in the cadastral area of Hronska Dubrava, Pitelova, Sasovske Podhradie and Ziar nad Hronom. We also documented damages caused by great cormorantfor fish in the river Hron. In the dissected great cormorants were found fish mostly fromthe carp family mainly the species of common barbel,common chub,European bleak and from theperch family it was European perch, other fish families occured rarely. At the end of the thesiswe discussed and evaluated the results, that we gained by exploration in the area.