We measured hemolymph Na and K concentrations, gut Na and K concentrations, transepithelial electrical potentials (TEPs) and fluxes of Na , K and water for the insectivorous praying mantis Tenodora sinensis (Orthoptera: Mantidae). In addition, we calculated transepithelial potential differences for Na (E_Na) and K (E_K). In the mantid, Na concentrations were higher in the hemolymph than in the crop, caeca, midgut, ileum, and rectum. Potassium ion concentrations were lower in the hemolymph than in the crop and rectum. All mantid TEPs were lumen negative. The crop TEP was less negative than the TEPs for the anterior and posterior caeca, anterior midgut, posterior midgut, and ileum. Mantid E_Na values were all negative and E_K values were all positive. Mantid caecal Na , K and water fluxes were all relatively small. These data imply that digestion of insect prey occurs largely in the crop, and the caeca and midgut may not play important roles in digestion. To maintain ionic homeostasis, mantids may actively transport Na while passively distributing K . We discuss these data for mantids in comparison to previous data on the gut function of desert locusts.