The frequency of micronuclei (MN) was determined in haemocytes of native mussels, Mytilus galloprovincialis, collected at five sites along the Eastern Adriatic coast in Croatia, four times throughout the year to evaluate seasonal variability in connection with possible genotoxic pollution. Chosen sites were either near closed industrial facilities or in tourist resorts. Mussels from a mussel farm were used as control. Seasonal variations in MN frequency were not observed for the control mussels, but for other investigated sites clear seasonal variations were observed. The highest frequency of MN was observed in summer on all studied sites, elevated in comparison to the control, while the lowest frequency was detected in autumn except for one site. These results pointed out that seasonal changes are observed only at polluted sites, most probably caused by seasonality of pollution as well as by interaction between contaminants and higher metabolic and filtration rates in mussels, resulting in higher values of cytogenetic damage.