Nephrocytes are said to be able to take up substances from the hemolymph. In Opiliones, which were examined electron microscopically three different types of nephrocytes were found. Numerous large nephrocytes lie clustered between the muscles in the anterior region of the body. Smaller nephrocytes occur scattered throughout the opilionid body, often affixed to tracheoles. The third group, pericardial cells, are always attached to the heart wall by connective ligaments. All nephrocytes are surrounded by a thick basement membrane and their plasma membrane forms pedicels. Junctional complexes link the adjacent pedicels to form diaphragm-like slit-membranes, which form the entrance to an extracellular compartment. The cytoplasm of the nephrocytes contains many pinocytotic vesicles and tubular elements. Different types of large electron-dense and electron-lucent vesicles can be distinguished. Occasionally a large bundle of unmyelinated nerve fibers is enclosed by a pericardial cell. Morphological differences between the types of nephrocytes are described and the ultrastructural characteristics of the nephrocytes of harvestmen are compared with those of other arthropods. Functional aspects are discussed with respect to their ultrafiltration structures.