Pinus canariensis is endemic to the Western Canary Islands, including the island of Tenerife with an area of 2034km2 and an a maximum altitude of 3715m. The temperature gradients in Tenerife vary from 0.43oC to 0.46oC per 100m, producing drastic temperature differences over only a few kilometres. With forest spanning an altitude from 500 to 2100m, Canary Island pines must either exhibit local adaptation or physiological plasticity to survive in such different environments. Canary Island pines possess a notable plasticity to regulate the proportion of the different photosynthetic pigments with the altitude to adapt to different levels of radiation. Also, the seedling survival rate under low temperature stress is higher for high altitude provenances suggesting genetic adaptations for different altitudes. If there are altitudinally adapted varieties one would expect an isolating mechanism in order to maintain them. We have examined the possibility of seasonal reproductive isolation in altitudinal transects of Canary Island pines in Tenerife. Flowering time is mainly determined by temperature and, thus, the flowering times at different altitudes are expected to be out of phase due to differences in temperature. Gene flow was analysed within and between two altitudinal transects with nuclear and chloroplast microsatellites.