The mucocyst ultrastructure in B. coli has not been described so far. As demonstrated in this work, cytoenzymatic assays on B. coli with the use of a reaction-detecting membrane-coupled hydrolase, i.e., ATP-ase, permitted identification of the mucocysts in the ciliate studied. The shape, size, and location of mucocysts in B. coli trophozoites were found to correspond to descriptions of these structures in other ciliates. The mucocysts were more numerous in B. coli trophozoites isolated from the symptomatic balantidiosis-affected pigs (Group I), and the product of reaction to ATP-ase was more copious than in Group II trophozoites. However, not all the bubble-like structures with similar morphological features reacted positively to the enzyme. The discrepancy was explained by the cytoenzymatic reaction to Beta-GR. The reaction product was visible in the vesicular structures, situated above the plasmolemma, although some of them contained no reaction product. Thus the presence of two types of secretory structure can be inferred: the mucocysts, with ATP-ase in their membranes, and other extrusomes containing active Beta-GR.