Phytoptus avellanae Nal. and Cecidophyopsis vermiformis Nal. commonly infest hazelnut plantations in the Blacksea Region. This study was performed on Tombul and Uzunmusa cultivars with periodic sampling during 1992-1994, in Samsun. Population fluctuations of the two species were investigated on big buds which contained either eggs, nymphs, adults of both species together or P. avellanae only. Although the numbers of P. avellanae in newly infested buds are very low in early summer, they reach maximum population levels during the following spring. C. vermiformis inside the summer big buds reaches the maximum population level in early autumn while C. vermiformis inside the winter big buds reaches the maximum population level in spring. In summer big buds infested by both species, the two populations are approximately equal in early summer whereas C. vermiformis becomes the dominant species from the end of July onwards. In winter big buds, although the number of C. vermiformis entering the big buds is low in autumn, C. vermiformis is usually the predominant population in spring.