Successful invasive species often exhibit high phenotypic lability. Even when intuition suggests that their genetic diversity has been reduced by strong founder effects, many successful invasive species still exhibit adaptive change in response to their new environment. To understand the probable long-term ecological impacts of a particular invader, we need to quantify long-term changes in the invader's phenotype, as these changes may well influence the level of impact the invader has on native species. Here we examine morphological change, as a consequence of time since colonisation and other spatial factors, in the cane toad (Bufo marinus). Cane toads are highly toxic and have killed many Australian native predators since they were introduced in 1935. The amount of toxin to which a predator is exposed will depend upon (1) the body size of the toad and (2) the relative toxicity of the toad (here measured by the relative size of the toad's parotoid glands). Using multiple regression and a model-selection approach, we show that both toad size and relative toxicity have decreased with time since colonisation. Thus toads (like many other successful invasives) exhibit phenotypic lability. Importantly, this result strongly suggests that toads will exert their maximal impact on native predators when they first arrive in an area; the level of impact will then decline over time.