Juvenile trees are expected to be more heavily defended against browsing mammals than mature plants. Juvenile and mature trees of Acacia tortilis and A. nilotica occurring at Nylsvley, Northern Transvaal, were quantitatively compared in terms o f some potential chemical and physical defences. Neither species showed any significant difference between juvenile and mature trees in terms of total polyphenol content, condensed tannin content, protein recipitating ability or protein content in leaves. Both species showed age-class differences in spinescence. In A. nilotica, thorns on branch tips were longer and more closely spaced and leaves were smaller in juveniles than in adults. Hence juveniles o f this species appear to be physically more heavily defended than mature plants. In A. tortilis, curved thorns were longer, but straight thorns were shorter than in mature trees. There was no difference between age classes in overall thorn density, but juveniles had a higher curved to straight thorn ratio. It is not obvious what the effects o f these differences might be on mammalian browsers.