This study was conducted to investigate the effect of the stem and bulb nematode Ditylenchus dipsaci Kühn (Tylenchida:Anguinidae) on yield parameters of some onion varieties in Karaman Province, Turkey. Experiments were established in a naturallyinfested onion feld during two consecutive years (2012 2013). Nematode population development was followed at the beginning andend of the experiment. Te reproduction rate was also calculated. Onion yield and the representative average diameter and length ofthe onion cultivar bulbs were determined. Yield parameters of six onion varieties were compared on nematicide-treated and untreatedcontrol plots in the experiments. Nematode populations signifcantly decreased in the nematicide-treated plots at the beginning ofthe experiment during both growing seasons. Te average population density of D. dipsaci was 104 individuals/100 g dry soil in thefrst year and 68 individuals/100 g dry soil in the second year of the experiment. Te initial and fnal nematode populations weresignifcantly positively correlated with Betapanko, Pan88, and Local variety1 cultivars, whereas they were negatively correlated withthe Valenciana cultivar during the frst year. Tis suggests that Betapanko, Pan88, and Local variety1 were susceptible to D. dipsaci,whereas the Valenciana cultivar could have some resistance. Te overall reproduction rate of D. dipsaci was negatively correlated withinitial population density and was primarily <1, which presumably represented a downward trend in nematode reproduction due toenvironmental conditions at the end of the experiment in September. Betapanko and Pan88 were susceptible varieties, and stem andbulb nematode caused yield losses of up to 12.93% and 15.33%, respectively; these varieties were considered intolerant. The yield ofLocal variety1 was positively related with initial nematode population density and it was categorized as tolerant to D. dipsaci.