The Mikomya coryli (Kieffer) (Diptera: Cecidomyiidae) is widespread in Europe and the most important cecidomyiid pest of hazelnut. Within-plant distribution, susceptibility of 18 Turkish hazelnut cultivars (‘Acı,’ ‘Allahverdi,’ ‘Çakıldak,’ ‘Cavcava,’ ‘Foşa,’ ‘İncekara,’ ‘Kalınkara,’ ‘Kan,’ ‘Karafindık,’ ‘Kargalak,’ ‘Kuş,’ ‘Mincane,’ ‘Palaz,’ ‘Sivri,’ ‘Tombul,’ ‘Uzunmusa,’ ‘Yassı Badem,’ and ‘Yuvarlak Badem’), and the phenology of larvae of this pest in hazelnut leaves and involucres were assessed in 2014 and 2015 in Giresun (Turkey). Mikomya coryli distribution differed significantly within different parts of the hazelnut plant. The highest gall numbers of M. coryli were found in the middle part (0.70–1.40 m) of the plant in both years. Total gall numbers varied between years: 1,779 and 2,588 galls were counted in 2014 and 2015, respectively. Susceptibility to M. coryli damage varied significantly among the cultivars. The highest leaf gall densities and total numbers of galls were found on leaves and involucres of Allahverdi, Yuvaklak Badem, and Yassı Badem cultivars in both years. Mikomya coryli larvae were detected between April and June in the leaf galls. The number of larvae in the involucres changed between April and mid-June. Results of the within-plant distribution, M. coryli larval phenology, and cultivar pest-susceptibility analyses are presented to enable effective control of the pest as a part of hazelnut integrated pest management.