Abstract: To understand the mechanisms underpinning the invasion of Alternanthera philoxeroides (Mart.) Griseb. in diverse water regimes, we compared its morphological traits, osmotic potential and methionine synthase (MS) gene expression with its native congener A. sessilis (Linn.) DC., which mainly occurs in moist terrestrial habitats in China. Results showed that A. philoxeroides exhibited greater phenotypic plasticity and possessed a more effective mechanism for osmotic adjustment than that of A. sessilis. The MS gene exhibited an ‘up-down’ expression pattern in A. philoxeroides: its mRNA levels increased during the first 3 hours of flooding treatments and then fell rapidly thereafter. Thus, higher flexibility in the internal environment and gene expression adjustment may play an important role in the rapid responses of A. philoxeroides to water regime changes, which are associated with its invasiveness.