Summary Many species of tardigrades are known to tolerate extreme environmental stress, yet detailed knowledge of the mechanisms underlying the remarkable adaptations of tardigrades is still lacking, as are answers to many questions regarding their basic biology. Here, we present data on the inorganic ion composition and total osmotic concentration of five different species of tardigrades (Echiniscus testudo, Milnesium tardigradum, Richtersius coronifer, Macrobiotus cf. hufelandi and Halobiotus crispae) using high-performance liquid chromatography and nanoliter osmometry. Quantification of the ionic content indicates that Na+ and Cl- are the principle inorganic ions in tardigrade fluids, albeit other ions, i.e. K+, NH4+, Ca2+, Mg2+, F-, SO42- and PO43- were also detected. In limno-terrestrial tardigrades, the respective ions are concentrated by a large factor compared to that of the external medium (Na+, ×70-800; K+, ×20-90; Ca2+ and Mg2+, ×30-200; F-, ×160-1040, Cl-, ×20-50; PO43-, ×700-2800; SO42-, ×30-150). In contrast, in the marine species H. crispae Na+, Cl- and SO42- are almost in ionic equilibrium with (brackish) salt water, while K+, Ca2+, Mg2+ and F- are only slightly concentrated (×2-10). An anion deficit of ~120 mEq 1-1 in M. tardigradum and H. crispae indicates the presence of unidentified ionic components in these species. Body fluid osmolality ranges from 361±49 mOsm kg-1 in R. coronifer to 961±43 mOsm kg-1 in H. crispae. Concentrations of most inorganic ions are largely identical between active and dehydrated groups of R. coronifer, suggesting that this tardigrade does not lose large quantities of inorganic ions during dehydration. The large osmotic and ionic gradients maintained by both limno-terrestrial and marine species are indicative of a powerful ion-retentive mechanism in Tardigrada. Moreover, our data indicate that cryptobiotic tardigrades contain a large fraction of unidentified organic osmolytes, the identification of which is expected to provide increased insight into the phenomenon of cryptobiosis.