The Lower Miocene fossiliferous bed of Canenx (Landes), accurately dated from its marine fauna, has yielded large quantities ofPinuspollen. Despite careful biometric observations, specific determination of these pollen grains is difficult, unless the ectexine structure of the air-sacks is taken into account. Light and scanning electron microscopy of the ectexine of the air-sacks of about 100, out of roughly 150 present-dayPinus-species, mainly American, has shown that the ectexine structure is always alveolate. The various features and parameters were found to be generally constant at species level. Using this new criterion, about 20 different forms could be distinguished among the fossilPinuspollen from the Canenx bed, 10 of which were identifiable to species level. Pollen of Asiatic, especially Chinese,Pinusspecies was poorly represented in our herbarium material, making the study of these floras difficult; however, the presence at Canenx of a few Asiatic genera is indicated, e.g.Cathaya,which nowadays grows in China. The study has provided fresh evidence for the existence of a large floristic province, the “Mixed mesophytic forest” of Graham (1972) in the Northern Hemisphere during the Middle Tertiary (Upper Oligocene and Miocene).