Allelopathy is an important biological phenomenon in exotic plant invasions. Studies about this phenomenon can help us to understand how plant interactions influence plant colony and ecosystem functioning. Both alligator weed (Alternanthera philoxeroides, Ap) and crofton weed (Eupatorium adenophorum, Ea) are important destructive exotic species in China. Their allelopathic effects on native plant species are well documented. However, whether alligator weed and crofton weed antagonize each other regarding plant growth? There is largely unknown currently. Here we report that (...) the leaf extract from crofton weed possesses the negative effect on the growth of alligator weed. The results showed that root growth of alligator weed could be seriously inhibited by aqueous extract from crofton weed, exhibited as no root or a few roots if only. Consistent with the inhibition of root growth, we observed changes of physiological and biochemical parameters in treated alligator weed. The chlorophyll content, the root activity and the acetolactate synthase (ALS) activity were significantly decreased; while the rate of superoxide anion (O2-), the malondialdehyde (MDA) content, the peroxidase (POD) activity, and the shikimic acid content were remarkably increased in the treated alligator weed plants. These physiological analyzes suggested that the main allelopathic effects of crofton weed were mainly through affecting the integrity of cell membrane and the activity of the key enzymes. Further, these data also imply that one exotic species has the potential value to be used in bio-control of the other exotic species. (shrink)