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Abstract: The effects of the Na+/K+ and Mg2+/Ca2+ ratios in saline groundwaters on Na+–K+-ATPase activity, survival and growth of Marsupenaeus japonicus postlarvae were investigated. The results indicate that the Na+–K+-ATPase activity, survival rate and weight gain of postlarvae were significantly affected by the Na+/K+ and Mg2+/Ca2+ ratios (P <0.05). The Na+–K+-ATPase activity of postlarvae, in every treatment, changed corresponding to Na+/K+ and Mg2+/Ca2+ ratios, and came to a stable level after 24 h. There was a negative relation between Na+–K+-ATPase activity and Na+/K+ ratio, while there was a positive relation between Na+–K+-ATPase activity and Mg2+/Ca2+ ratio. Compared with seawater (the Na+/K+ and Mg2+/Ca2+ ratios are 27.8 and 4.64 respectively), the Na+–K+-ATPase activity of the Na+/K+ ratio 30 treatment showed no significant difference, while the Mg2+/Ca2+ ratio 4.5 treatment showed distinct difference. The survival rates and weight gain of postlarvae increased markedly when the suitable amount of K+ and Ca2+ was added to test water, and arrived at their maximum in the Na+/K+ ratio 20–30 or Mg2+/Ca2+ ratio 4.5 treatment, having no significant difference compared with normal seawater. Therefore, considering the Na+/K+, Mg2+/Ca2+ ratios and the absolute concentration of Mg2+, Ca2+ in the experimental saline groundwaters applied to Marsupenaeus japonicus farming, it should be modulated to around 30, 4.5 and 1312 mg/l, 291 mg/l, respectively. [Copyright &y& Elsevier]