In this paper we studied the influence of polychaete infestation of the genus Polydora on the meat and shell quality of the European flat oyster Ostrea edulis (Linneaus, 1758) and the Pacific oyster Crassostrea gigas (Thunberg, 1793) sampled from Medulin Bay. Oyster samples were collected randomly at three stations and at different depths along the area of Medulin Bay. For the efficient extraction of polychaetes, after morphometric tests and inspection of the inner shell, the oysters were placed in a 2M solution of hydrochloric acid in order to quantitatively determine the degree of infestation for each oyster. The results of this research indicate that oysters from Medulin Bay in all three stations can be classified as the unclassified category in terms of meat quality. Considering the degree of damage to the inner shell, the wild C.gigas and cultured O. edulis populations were mostly classified as excellent, while the wild O. edulis population showed the greatest variability in shell damage between the tested oysters, where 50 % of the oysters belonged to commercial unacceptable III and IV category. The percentage of oysters with visible damage on the inner surface of the shell caused by the polychaete of the genus Polydora was determined at 100% for cultivated and wild populations of O. edulis, while a lower percentage of infestation was recorded for the population of C. gigas (90%). Polydora index values ranged from 0.23 to 0.62 and differed significantly between stations. The minimal impact of polychaetes on shell quality was recorded in the wild population of C. gigas (PI=0.23), while the maximum impact on shell quality was recorded in the wild population of O. edulis (PI=0.62). The average number of parasites between samples ranged from 4-15, while 28 was the highest recorded number of polychaetes in an oyster. The research determined that polychaete infestations of the genus Polydora do not have a significant effect on the meat index of oysters from Medulin Bay. Considering the small number of polychaetes found in the shells, and the poor meat index that prevailed in most samples, it was associated with the spawning season that was in progress during this research. Significant differences were recorded between the selected sampling stations regarding the meat index, visible shell damage, PI and mean number of parasites, which is a possible consequence of different hydrographic parameters present at each station. During the research, cultivated and wild populations of O. edulis showed greater susceptibility to infestations by species of the genus Polydora compared to the wild population of C. gigas.