Pacific oyster Magallana gigas (Thunberg, 1793) appeared in the Adriatic Sea in the late 1970s. Although it is cultivated around Europe, in the eastern Adriatic Sea this species was not introduced to the aquaculture, and it is considered invasive. The wild population of the Pacific oyster is present in great abundance in the Lim Bay, which is one of the most important aquaculture sites in the Adriatic Sea. In the Lim Bay, only two bivalve species are cultivated: oyster Ostrea edulis (Linnaeus, 1758) and mussel Mytilus galloprovincialis (Lamarck, 1819). In this area, there is no information about possible effects that the Pacific oyster could have on native species, which can be expressed through the competition for food. Therefore, the main objective of this thesis was to provide a comparative study of feeding habits of two oyster species, M. gigas and O. edulis. In situ experiments of different physiological parameters were carried out in April and August 2018 while stomach content analysis was carried out during each season (February, May, August and November 2018). Results of in situ experiments suggested an overlap of the food niche between two investigated species under the conditions prevailing in Lim Bay. Stomach content analysis showed that both oyster species used zooplankton as an additional food source. The most abundant zooplankton prey groups found in both species were bivalve larvae and copepods. Furthermore, numbers of bivalve larvae found in an adult O. edulis was 720 larvae, which is so far the highest recorded for any bivalve species, while the maximum number of bivalve larvae in the stomach of M. gigas was 443 larvae. Results of this research confirmed a high degree of feeding niche overlap between M. gigas and O. edulis. Furthermore, this study revealed a high degree of larviphagy in both oyster species.