The annual cycle of adult butterflies was studied at seven rocky outcrops located along an altitudinal gradient in a northern Spanish mountain area (Picos de Europa) with a Eurosiberian climate in 1993 and 1995. Both total abundance and number of species of butterflies peaked in summer at all study sites. The cumulative species number uniformly increased over the season until late summer. These results imply that butterflies concentrated their activity when temperatures were higher in the year. Regardless of their diversity and total abundance, timing of the flight period was later for assemblages at high altitude than for assemblages at low altitude. A similar trend in increasing delay in flight period was found when data were analysed separately for each species. The duration of the flight period of assemblages was correlated with abundance in 1993, but it was correlated with altitude in 1995. Most species tended to show shorter flight periods at high altitudes in both study years. These altitudinal changes in timing of adult flight are likely to be linked with effects of temperature on developmental rates of early stages of the life cycle. Changes in duration of the flight period, however, might be partly related to variability in species abundance.