1. 1. The study was carried out on 22 species of insects from 5 orders. The osmolality of their hemolymph varied from 319 to 421 mOsm/kg H 2O, concentration of Na + 4.6 to 118 mM/l, K + 6.3 to 73mM/l, Ca 2+ 3.6 to 12.9 mM/l, Mg 2+ 2.3 to 76 mM/l. The most abundant cation in the hemolymph of insects from higher orders is either K + or Mg 2+. 2. 2. In the muscles of lower and higher insects K + is usually within 80–120 mM/kg wet wt. 3. 3. Most Ca 2+ and Mg 2+ in hemolymph is bound with protein and low molecular anions, concentration of free Ca 2+ is 0.9-2.1mM/l Mg 2+ 3.7–8.0 mM/l. 4. 4. It is concluded that, in insects, potassium hemolymph, cell volume regulation and accumulation of ions in the cell, are ensured by an increased osmolality of hemolymph due to a high percentage contribution of low molecular organic substances which are retained in the hemolymph due to the absence of filtration apparatus in the Malpighian tubules.