The degree to which physical defence mechanisms are present in toxic species of Gastrolobium was compared with the known fluoroacetate (the toxic principle) concentrations of these plants using both histological leaf sections prepared from fresh leaves (4 species), and a variety of visual external traits measured from herbarium specimens (28 species). There was a strong negative correlation between the presence of physical deterrents (e.g. area of fibres, number and length of spines) and the fluoroacetate concentration of each species. This suggests that, with respect to their leaves, individual species have established a compromise between producing physical grazing deterrents and the adoption of chemically mediated antiherbivore strategies.