In this study, the aim was to investigate the seasonal variations in the selectivity of striped red mullet (Mullus surmuletus L., 1758) captured by gill nets during the year in the north Aegean Sea. Fisheries' operations were carried out between March 2008 and August 2009. Gill nets with 18, 20, and 22 mm mesh sizes were used in the trials. These nets are widely used in the region. The SELECT method was used to determine the selectivity of gill nets. The deviances from the SELECT method revealed that lognormal models gave the best fits for all seasons. As a result of the calculations made according to the lognormal model, the modal lengths of the gill nets were calculated as the shortest in the spring season and the longest in the summer. The spread values of the selectivity curves of the experimental gill nets were determined the lowest in the winter season and the highest in the summer season. As a result of selectivity analysis, the differences between the spring and summer season modal lengths of the nets were calculated as 1.5 cm, 1.65 cm, and 1.82 cm for the nets with 18 mm, 20 mm, and 22 mm mesh sizes, respectively. The study results showed that the fishing season was important in gill nets selectivity.