This study aimed to determine the levels of various polycyclic aromatic hydrocarbon compounds in Mytilus galloprovincialis, seasonally obtained from nine stations in the Istanbul Strait, and their effects on antioxidant enzyme activities. Samples were sorted into two groups based on length as 3-5 cm and 5-7 cm, and all analyses were carried out separately for both groups. Salinity, dissolved oxygen, temperature and pH of sea-water at sampling stations were measured. Very high levels of PAHs were detected, especially in Spring and Summer, hence, the risk of consuming mussels in those seasons was indicated. Mean total PAH was highest in Summer (617,86 ng/g), followed by Spring, Winter and Autumn (536,77 ng/g, 293,34 ng/g, 203,70 ng/g, respectively). The level of Benzo(a)pyrene, potentially the most carcinogenic compound among PAHs, was four times higher than the allowed limits of the European Union. Also, the total level of four PAH compounds (Benzo(a)pyrene, Chrysene, Benz(a)anthracene, Benzo(b)fluoranthene), considered as an indicator of carcinogenic risk, was three times higher than the allowed limits. Catalase, peroxidase and superoxide dismutase enzyme activities were calculated. Mean values of catalase and peroxidase activities were highest in Autumn, followed by Summer, while superoxide dismutase activity was highest in Summer, followed by Autumn. The lowest level of enzyme activity for all three enzymes was in Spring. Positive correlations between enzyme activities and PAH compounds were observed only in Winter and Spring. Catalase was positively correlated with six different PAH compounds, peroxidase with five different and superoxide dismutase with two different PAH compounds. The highest number of negative correlations between enzyme activities and PAH compounds was detected for superoxide dismutase, which was negatively correlated with total PAH and seven different PAH compounds, most of which were observed in Autumn.