Abstract: The dominance of Paspalum vaginatum in mangrove swamp at the University of Lagos, Nigeria, was investigated by determining its growth and physiological responses to various salinity levels. P. scrobiculatum which occurs further inland was included for comparison. Uniform cuttings of each species were subjected to salinity levels of 0-400 mM NaCl using artificial seawater in one-fifth strength Hoagland solution in a glasshouse for 28 days. The results showed that P. vaginatum behaved like a halophyte showing enhanced growth at low salinity and tolerating high salinity with no significant decrease in water content but with significant increase in chlorophyll content as salinity increased. On the other hand, P. scrobiculatum behaved like a non-halophyte with decreased growth and significant reduction in water content but with significant increase in chlorophyll content as salinity increased and is incapable of tolerating high salinity. The results indicate that P. vaginatum tolerated salinity by regulating the amount of salt in the shoot while retaining enough water for osmotic adjustment. P. scrobiculatum could neither regulate salt uptake, nor retain enough water for osmotic adjustment. It was concluded that these responses may be partly responsible for the ability of the species to inhabit their different environments.