The ultrastructural changes in the tracheal epithelial cells of chicks with experimental cryptosporidiosis were examined. There was deciliation and formation of compound cilia whereas the microvilli showed Y-shaped formation, atrophy or blunting, fusion and branching. The basal bodies and striated rootlets were irregular and sometimes wanting while there was an increase in centrioles beneath the cilia. Nuclear atypia was common whereas the nucleoli were hypertrophic with reticular nucleolonema. Double nucleoli were mostly reticular and rarely ring-shaped or of thin-shelled type. There was an increase in well-developed rough endoplasmic reticulum and Golgi complex, ribosomes, coated vesicles, smooth-membraned vacuoles, mitochondria and mitochondrial-RER associations. The smooth and rough endoplasmic reticula had dilated cisternae. Nuclear bodies, triple nucleoli, nucleolar margination and segregation, ring-shaped and thin-shelled nucleoli, cytoplasmic imagination, autolysosomes, multivesicular bodies, electron-dense fibres and vesicular rough endoplasmic reticulum were rare. Cryptosporidium sp. attached even to the goblet cells, which confirmed that the microvilli played an important role in the attachment process of the parasite. The surface changes of the cilia and microvilli may denote either an adaptation or compensatory response to cryptosporidiosis. The major changes, however, indicated that the tracheal epithelial cells have the same capacity to increase their protein synthesis as the chicken bursae, in response to the parasitism.