Abstract This study was carried out to evaluate the phenological changes resulting from external impacts on Posidonia oceanica meadows along the Aegean coast. Triplicate samples were collected from 17 stations at 8 and 10 m depths by SCUBA diving between April and June 1999. The water temperature was measured and sediment type was determined in the stations. The phenological parameters; shoot density, leaf number, leaf area index, leaf surface area, brown tissue surface area and total biomass were investigated. One of the most important descriptors of Posidonia, shoot density, showed supranormal values at the seven of the meadows while only two of them showed abnormal values. Beside the natural factors such as water temperature and sediment types, the possible environmental or antrophogenic impacts on phenological characters of Posidonia meadows were discussed.