The cytoskeleton of Trichoplax adhaerens fiber cells was studied after chemical fixation, freeze-substitution, lysis of attached cells with nonionic detergents and by immunofluorescence. Cytoskeletal elements present in the cell bodies and reaching into the extensions include microtubules, intermediate filaments, 6–7 nm and 2–3 nm microfilaments. The latter seem to interconnect other cytoskeletal elements. Actin-like microfilaments are found both as networks and parallel strands. Immunofluorescence with antiactin shows the presence of actin in the cell body, underneath the plasmalemma and within the extensions. Both the results of immunofluorescence and the identification of 6–7 nm actin-like microfilaments support the concept of contractility of the fiber cells as the cause of the rapid shape changes of Trichoplax. Anti-tubulin fluorescence corresponds to the location of microtubules in the extensions as well as the cell bodies of the fiber cells. The extensions are withdrawn upon depolymerization of the microtubules by colchicine.