Fifteen populations of Azolla were sampled in the western and north-eastern parts of The Netherlands. After cultivation under identical conditions in a greenhouse they could be divided into two species which had previously been recorded from The Netherlands under the names of Azolla filiculoides Lam. and Azolla carotiniana Willd. Populations which appeared to be morphologically intermediate between these two species were eventually found to be true A. filiculoides. Although identification of certain forms in the field remains difficult, the two species can always be distinguished under a microscope by the number of cells forming the trichomes on the surface of the upper lobe of the leaflets, the septation of the glochidia and the surface structure of the megaspores. Other characteristics which have been reported in the literature are not always consistent. However, well-developed specimens of A. filiculoides can easily be recognised from their general appearance. In the autumn, when the plants become reddish as a result of anthocyanin production, A. filiculoides turns to a purer red than the plants of the other species which become more browny red. In both species the anthocyanins contain the relatively rare anthocyanidins luteolinidin and apigenidin. Anthocyanin production was found to be influenced by temperature and water composition but not by day-length. Well-developed forms of A. filiculoides generally occur in more eutrophic water than plants of the other species. According to the revised classification of the New World species of Azolla by Svenson (1944), the Dutch specimens previously referred to as A. carotiniana do not belong to this species, but to either A. mexicana Presl or A. microphylla Kaulfuss.