AbstractBetween 2005 and 2007, the presence ofOnchocerca flexuosa(Wedl, 1856) was discovered and investigated in 110 red deer (Cervus elaphus) shot in the Riaño Regional Hunting Reserve, in the province of León (north-western Spain). Nodules containingO. flexuosawere located in the dorsal region and flanks of the deer. These were collected and measured, and some adult parasites were extracted from the nodules and identified by morphology and by obtaining mitochondrial 12S rDNA sequences, which were identical to those of previously published sequences forO. flexuosa. Some nodules were prepared for histology, embedded in paraffin, sectioned and stained with haematoxylin–eosin. Histologically, the worms were found in several compartments separated by an infiltrated fibrous tissue. These compartments were inhabited by several females and males, surrounded by a fibrous capsule. A total of 85.45% (95% confidence interval (CI): 78.86–92.04%) of red deer were parasitized, with a mean intensity of 9.53 ± 12.27 nodules/host, ranging between 1 and 74 nodules/deer. Significant differences in prevalence and intensity of infection were found between young and adult red deer, and also between seasons. However, no significant differences between males and females were observed. Five hundred and ninety-seven nodules were measured (15.81 ± 3.94 mm) and classified by sizes into small ( < 10 mm), medium (10–20 mm) and large (>20 mm). No relation was found between the size of the nodules and the time of infection. The high values found in the studied parameters show that northern Spain is an area of high-intensity infection for deer.