Ball et al. identified that the concentration of K+ in leaves of Avicennia marina decreased under high salinity conditions in a culture solution. With this K+ decrease in the leaf, the photosynthetic capacity declined. This effect suggests the concentration of K+ in leaves and the soil water salinity are limiting factors in mangrove vegetation. We examined this hypothesis using A. marina, Rhizophora stylosa and Bruguiera gymnorrhiza in this study. This study was done in the Republic of Djibouti where the pure forest of A. marina is widely distributed and on Iriomote Island in Okinawa where R. stylosa and B. gymnorrhiza are distributed. The relationship between the concentration of K+ in leaves and the soil water salinity in R. stylosa and B.gymnorrhiza revealed no significant correlation. However, this relationship in A. marina revealed a significant correlation. And then, the relationship between concentrations of Na+ and K+ in leaves in R. stylosa and B. gymnorrhiza revealed no significant correlation. However, this relationship in A. marina revealed a significant correlation. These results may suggest that the effect caused by the concentration of K+ in leaves and the soil water salinity in A. marina is a limiting factor in the distribution of A. marina, but that hypothesis cannot be applied to R. stylosa and B. gymnorrhiza.