1. Analyses were made of haemolymph obtained from aquatic representatives of nine major orders in the Insecta.
2. In the Odonata, Plecoptera, Ephemeroptera and Hemiptera-Heteroptera, the sodium concentration is relatively high and the potassium concentration is low. The chloride concentration is relatively high, but it never exceeds the sodium concentration.
3. In the Neuroptera, Coleoptera, Trichoptera and Diptera the sodium concentration is relatively high and the potassium concentration is low. In Nymphula nymphaeta (Lepidoptera) both the sodium and potassium concentrations are low. In all of these endopterygotes the chloride concentration is relatively low.
4. There is no correlation between diet and the sodium/potassium ratio in haemolymph of aquatic insects.
5. The problem of cation-anion balance is discussed. It is suggested that free amino acids contribute little to the anion pool. Bicarbonate and phosphate, largely as inorganic phosphate, contribute a small amount to the anion pool in exopterygotes. Phosphate contributes considerably more to the anion pool in some endopterygotes. Nevertheless, in all of the insects investigated, there is a deficit of anions. This deficit is usually greater in the endopterygotes.
6. The calculated osmotic activities of the known ions and free amino acids are less than the observed osmotic activities of haemolymph. It is inferred that non-amino organic acids must be present. Preliminary investigations indicate that non-amino acids are present in the concentrations required to account for both the osmotic and the anion deficits.