The seasonal dynamics of ticks from sheep in the Pais Vasco (North of Spain) and the correlation with the climatic data were studied. Haemaphysalis punctata Iarvae display a very short seasonal activity, lasting from August to mid-September, with no significant correlations to climatic variables. The nymphs of this tick species follow a bimodal activity pattern, clearly in correlation with maximum temperature and rainfall. The adults are present during the cool-wet and warm-wet seasons and their activity is correlated to minimum air temperature. Adults of Rhipicephalus bursa showed also a bimodal seasonal pattern which extends July to September. Dermacentor marginatus is present only the late-winter (February) until mid-April. Both R. bursa and D. marginatus showed so many significant correlations as to attribute the regulation of activity to any group of factors.