A number of large-growing species of Agave L. have become naturalised in southern Africa. One of these, the blue-leaved Agave americana L. (subsp. americana) var. americana, can take on colossal proportions. Agave weberi J.F.Cels ex J.Poiss. has a similar general appearance and also attains very large rosette and inflorescence dimensions. It is often confused with A. americana, and has comparable invasive tendencies. We give the differences between these two species, and provide an amplified description of A. weberi. For the first time we illustrate the flowers of this species, which often produces an inflorescence of over 10m tall. Nomenclatural information is provided on Agave franceschiana Trel. ex A.Berger and Agave neglecta Small, synonyms of Agave weberi.