We observed a carcinoid of the thymus that was remarkable for the presence of intracytoplasmic inclusions-readily visible by light microscopy-in the majority of tumor cells. The inclusions were best seen in Grimelius-stained preparations, which also revealed argyrophilic granules present within or close to the masses. On electron microscope examination the inclusions were found to be composed of aggregates of filaments, probably of the intermediate type, which were at times accompanied by neurosecretory granules. The observed incompatibility between the presence of such inclusions and a homogeneous distribution of secretory granules in the cytoplasm is suggestive of a dysfunction of intracellular movements. Microfilamentous carcinoids may represent a class of tumor having a pathologic excretory process.