One of the best documented effects of climate change on biodiversity are shifts in phenology. However, long-term data quantifying and projecting the expected changes in phenology associated with climate warming are limited to a few well-recorded areas in the world. In the absence of temporal recording, an alternative approach is to determine the phenological response of species along marked gradients in climate or along latitudinal or altitudinal transects (space-for-time substitution). We studied the phenology (timing and duration of the flight period) of butterflies in 2006 along an altitudinal gradient (900-1680 m; estimated temperature lapse rate = -6.6°C/km) in the Serranía de Cuenca (central Spain) at the assemblage and individual species levels. Timing of the flight period was later for assemblages at high than at low altitudes. A similar trend of an increasing delay in the flight period with altitude was recorded for some individual species. However, there were also some exceptions to this pattern regardless of the number of sites and the altitudinal ranges of the species, suggesting possible local adaptation to regional climate. The duration of the flight period was shorter at high altitudes for assemblages, but this trend was not mirrored in the response of individual species. The results partly support substituting space-for-time when assessing the potential effect of climate change on phenophases such as the timing of the flight period, but we recommend extreme caution in extrapolating the results in the absence of information on how the responses of populations differ. and Juan Ignacio De Arce Crespo, David Gutiérrez.