Onchocercosis, caused by Onchocerca flexuosa, was observed in red deer (Cervus elaphus) from Spain for the first time. Adult specimens of O. flexuosa were found in nodules in subcutaneous tissues in 42 of 125 (33%) red deer between October 1994 and September 1995; intensity of infection ± SD was 3.93 ± 5.26 nodules per infected host. A clear seasonal pattern in the distribution of nodules was observed, with higher values of prevalence and intensity in fall and winter in contrast to spring and summer. Significant differences were found among age groups in prevalence, but not in the mean intensity. No differences in infection were apparent between male and female adult red deer.