The excretory organ of terrestrial mites Leptotrombidium orientale (Schluger, 1948), Euschoengastia rotundata (Schluger, 1955), and Hirsutiella zachvatkini (Schluger, 1948) (Trombiculidae); Platytrombidium fasciatum(Koch, 1836) and Camerotrombidium pexatum (Koch, 1837) (Microtrombidiidae) as well as of water mites Teutonia cometes (Koch, 1837) (Teutoniidae) and Piona carnea (Koch, 1836) (Pionidae) in different developmental stages was studied using transmission electron microscopy and on semi-thin sections. Irrespectively of the species studied and developmental stages, the excretory organ is represented by a simple thin-walled blind sac that occupies an axial position. The excretory organ is not divided into obvious morphologically or functionally different parts. Posteriorly, the excretory organ transforms into the short cuticular-lined excretory duct, the proper hindgut, derivative of proctodeum. The excretory duct terminates by the axially orientated fissured opening (excretory pore, uropore) that anatomically corresponds to the anus, located on the ventral body wall. In larvae P. carnea, the excretory duct and the excretory pore are lacking. The lumen of the excretory organ is variously filled with yellowish double-refractive crystals of excretory wastes (probably guanine) and sometimes also contains single electron-dense globule. The walls of the excretory organ are formed of the single-layered endodermal epithelium, composed of the uniform cells greatly variable in their shape and size. The basal plasma membrane forms round digital indentations frequently containing corresponding projections of the underlying connective tissue cells penetrating through the basal lamina. Clear vacuoles resulting from these indentations immerse into the cells and migrate through the cytoplasm to the apical plasma membrane where they are discharged from the cells. The cells may also contain round or irregularly shaped electron-dense bodies resembling secondary lysosomes or residual bodies. These inclusions may come close to the apical plasma membrane and may be release from the cells. The apical plasma membrane forms irregular microvilli as well as larger irregular projections also bearing microvilli. Golgi bodies and profiles of granular endoplasmic reticulum occur only rarely. Passive transport of residual precursors, vacuolar transport as well as excretion of the electron-dense material provides the accumulation of wastes in the lumen of the organ.