The tegument ultrastructure of the cestode Triaenophorus nodulosus has been studied in the stages of oncosphere, procercoid, plerocercoid and adult. The syncytium of primary tegument has glandular origin and is located in the peripheral areas of the oncosphere. The primary tegument degenerates at the initial stages of the procercoid development and is replaced with secondary tegument persisting throughout all following stages of the worm’s development. Two ways of microthrix formation on the body surface of procercoid were discovered. The formation of the cyst consisting of fibrillar material around the plerocercoid was observed. It fills spaces between numerous finger-like évaginations of plerocercoid’s tegument. The structural differentiation of tegument and microtriches was demonstrated on the scolex and all parts of strabila of T. nodulosus.