We investigated the relationship between ion concentration of dimorphic seeds and salt tolerance of young seedlings from dimorphic seeds, and the effect of salinity on seedling growth, ion content and photosynthetic characteristics in leaves of plants from dimorphic seeds of Suaeda salsa L. in sand culture. concentrations of Na⁺, K⁺, Cl^- and total ions of brown seeds were 2.3, 1.5, 1.8 and 2.0 times higher than those of black seeds, respectively. Compared to 0 mmol/L NaCl, 100 and 400 mmol/L NaCl did not inhibit the elongation of seedlings from brown seeds, but the opposite trend was found for seedlings from black seeds. The elongation of seedlings from either brown seeds or black seeds markedly decreased in 800 mmol/L NaCl. Compared to 0 mmol/L NaCl, shoot dry weight significantly increased in 200 mmol/L NaCl, whereas 600 mmol/L NaCl decreased shoot dry weight of plants from dimorphic seeds. Lower salinity (200 mmol/L NaCl) had no effect on the number of the side branches along the main stem of plants from dimorphic seeds, while higher salinity (600 mmol/L NaCl) markedly decreased this number in plants from dimorphic seeds, especially for plants from black seeds. Shoot dry weight and the number of side branches along the main stem of plants from brown seeds were much higher than those of plants from black seeds under salinity. High salinity (600 mmol/L NaCl) decreased the rate of photosynthetic oxygen evolution in leaves of plants from dimorphic seeds. Salinity (200 and 600 mmol/L NaCl) markedly decreased concentrations of chlorophyll a and chlorophyll b in leaves of plants from black seeds, while 200 mmol/L NaCl had no adverse effect on these parameters in leaves of plants from brown seeds. Leaf Na⁺ concentrations increased with increasing concentration of NaCl, whereas leaf K⁺ concentrations decreased with increasing concentration of NaCl for dimorphic seeds. There was no significant difference in ion concentration, concentration of chlorophyll and the rate of photosynthetic oxygen evolution in leaves of plants from dimorphic seeds of S. salsa. The present results indicate that higher concentrations of ions in brown seeds and better development of brown seeds may be related to higher salt tolerance of seedlings from brown seeds, and more branches may be related to higher biomass of shoots for seedlings form brown seeds. These characteristics of dimorphic seeds of S. salsa may affect their roles in establishment of the population in saline environments. Brown seeds may play a main role in seedling establishment in spring, while black seeds may play a sustaining role in seedling establishment under unfavourable conditions.