A study was estabilished for investigation of nematodes in onion growing areas in Karaman province in Turkey by support of the Turkish Scientific and Technical Research Foundation in 2011 for three years. The first step of the study was to investigate the distribution of the nematodes in onion growing areas in Karaman province where has an important role in onion production in Turkey. Ditylenchus spp. (Filipjev 1936) (Tylenchida: Anguinidae) were in 15% of plants and 61% of soils. Other widespread plant parasitic genera were Paratylenchus (Micoletzky 1922) (Tylenchida: Paratylenchidae) (56%) and Tylenchus (Bastian 1865) (Tylenchida: Tylenchidae) (49%). Besides fungivorous and bacterivorous nematode genera were also abundantly found. Second objective of the study was to estabilish the yield loss by stem and bulb nematode found in a farmer field. Study was inferred up to 40% yield loss duo to the stem and bulb nematode. The most suitable and less labored method to keep the nematode populations under damage level is use of the resistant cultivars. Resistance reaction of commercially produced onion cultivars and breeding material in Turkey was investigated under controlled conditions. The cultivars of Imrali Kirmasi, Karaca, Banka, Mor Sogan Tohumu, Aki and Kirehir local population (Gen Bank No: TR69450) were found hope giving for resistance against D. dipsaci.