We hypothesize that salinity stress may leave a specific pattern of δ13C variation in common reed (Phragmites australis (Cav.) Trin. Ex Steudel), a species potentially susceptible to salinity. We measured dry weight and δ13C of P. australis transplanted in a constructed saline wetland four times between July 2001 and May 2002. Shoot dry weight (590–3090 g m−2) of P. australis was negatively correlated with rhizosphere salinity. Shoot δ13C values (−27.1 to −23.8‰) in the saline wetland were less negative than that of control samples (−28.7 to −28.2‰) collected from non-saline riparian zones nearby. In addition, the δ13C values became less negative, indicating decreased carbon isotope discrimination, as salinity increased. These results are consistent with published gas exchange measurements that showed that salinity stress could decrease CO2 diffusion rate by reducing stomatal and/or mesophyll conductance. However, we could not preclude the possibility of growth reduction caused by salinity effects on carboxylation. We suggest that the non-destructive and time-integrating δ13C measurement is a useful tool in monitoring responses of P. australis to environmental conditions in saline wetlands.