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