Exposure to a hypo- or hyper-osmotic medium imposes a concentration gradient between the environment and the animal. Unless efficient mechanisms of ion and water regulation are in place, then the exposure of fish to such conditions will result in changes in plasma osmolality. Plasma osmolality is therefore a useful marker to determine the effectiveness of osmotic regulation. Previous studies in an Australian population of inanga have shown that plasma osmolality is maintained constant at a value of ~ 268 mOsm kg− 1 from FW to about 20 salinity, and these authors classified inanga as a “powerful osmoregulator”. A recent study reported plasma osmolalities of 274 mOsm kg− 1 in inanga maintained in FW, a value that increased to 357 mOsm kg− 1 after 8 h of challenge with water at 28 salinity. In the same study, the authors reported that after 48 h plasma osmolality reached a “new seawater equilibrium”, at a value of 312 mOsm kg− 1. In the present study plasma osmolality was 301.3 ± 0.6 mOsm kg− 1 in FW and 336.1 ± 5.1 mOsm kg− 1 at a salinity of 33, indicating good regulation over this range. However, at the highest salinity (43) plasma osmolality reached significantly higher values (411.1 ± 14.2 mOsm kg− 1). This, however, does not necessarily means that the fish was experiencing an ion imbalance, but may simply signal a new equilibrium. An increase in plasma osmolality in fish acclimated to higher salinities is a common phenomenon, as it is thought to minimise the concentration gradient for ion influx. Consequently this elevated plasma osmolality value observed at 43 salinity, may simply represent the top end of the optimal range of plasma osmolalities in inanga. Future studies using higher salinities are needed to determine the upper limit of salinity tolerance/regulation in inanga.