The stomatal types of 102 species illustrating the different subdivisions of Bentham's classification of the genus Acacia were studied at two or three ontogenetic stages: first pinnate leaf to bipinnate leaves or to phyllode. Six stomatal types are recognized on the basis of Guyot's nomenclature (1966): 1, 2, 3, 4, 4′, 6; a new type (3′) is described. The specialization of the leaf up to the phyllode stage is followed by a decrease of diversity of the stomatal formula and an increasing frequency of the ‘basic core’ 3, 3′, 4. The distinction between the two Australian groups of bipinnate species (series Pulchellae and Botrycephalae) is confirmed. Possible relationships between the cosmopolitan series Vulgares (the most primitive group) and some Australian taxa as well as between the Pulchellae and some phyllodic species from the Australian series Phyllodineae are commented on. The series Gummiferae looks quite distinct from other groups.