The olive fruit fly, Bactrocera oleae (Gmelin) (Diptera: Tephritidae), is one of the most important olive pests in Turkey, as well as throughout the olive growing regions in the world. The aims of the study are to determine infestation rate of B. oleae and to asses the effects of pest infestation on yield, hundred fruit weight, maturation index, acidity and fatty acid composition of olive oil at different harvest times. This study was carried out in five selected olive orchards in Kucukkuyu-Ayvacik (Canakkale), representing the Edremit Gulf Region, where aerial spraying is not being applied. Only trees representing cv. Ayvalik, the main cultivars of the region, were selected for the experiment. Adults were monitored with yellow sticky traps from the beginning of July to the end of December in 2005. Samples were collected at five different dates in the five orchards. The results showed that the level of B. oleae adults population was low during July and August and reached the highest level at the end of October. The infestation rates of B. oleae at all harvesting dates were observed parallel to the adult population densities in all five orchards. The puncture rate considerably increased after mid November, therefore, early harvest definitely contributed to the preventing the fruit from B. oleae infestation. There were no statistical differences in hundred fruit weight among different harvest times. Oil yield increased rapidly until 10th of November. After that time, the increase rates gradually stopped and there were no statistical differences between oil yields. Therefore, fruit harvest should be carried out starting from mid-November to obtain the highest oil yield and avoid higher infestation rates. It is obvious that the olive oil acidity is affected by B. oleae infestation but in presented study, the results haven't complied with this due to a considerable percentage of fly-damaged fruits that would cause high acidity dropped prior to early November. The fatty acid composition was affected by olive ripening. Although it was effectively proved through previous research on the fatty acid composition, the effects of B. oleae infestation haven't been determined because the analysis was carried out on the fruits hanging on the trees as suggested by international Olive Oil Council (IOOC). According to the results the harvest should be done when olives are turning purple or have purple skin for minimizing the damage caused by B. oleae. Regarding to the above criteria, the suitable harvest time was found as the mid of November, when fruit maturation index is between 2.5 and 3.5. Results indicate that the B. oleae infestation could be significantly reduced by the implementation of early harvest.