During 1992-1994 we have investigated in detail the life cycles of the two forms of Phytoptus avellcmae Nal. ("gall" and "vagrant" forms) and of Cecidophyopsis vermiformis Nal. which cause serious worldwide damage in hazelnut cultivars. Selected hazelnut plantations from the Blacksea Region of Turkey were visited every 2-3 days during the migration period, once every two weeks during the winter months and weekly for the rest of the year and samples were collected. The gall form of P.avellanae has a single nymph form and a simple life cycle. In spring, nymphs migrate from old big buds to healthy buds where feeding and reproduction continues until the following spring. Since the entire population is stationary within the same bud throughout the year, generations overlap. The vagrant form of P.avellanae has a different and rather complex life cycle and two forms of nymphs are present. The vagrant form feeds not only on big buds but also on vegetative and generative parts such as leaves, male and female flowers and completes its life cycle even in the absence of big buds. C.vermiformis also has a complex life cycle with two migration periods in spring and autumn. It lives inside the big buds created by P.avellanae and its generations overlap. C.vermiformis is rarely found on male and female flowers and vegetative buds.