Abstract: Giardia lamblia trophozoites were cultivated axenically in TYI-S-33 modified medium containing 1.345 mg/ml of osthole(24 h IC50）. The parasites were observed by scanning and transmission electron microscopes after treated with osthole for 24 h. The surface of the trophozoites treated with osthole was rough. The surface of ventral sucker and median body had obvious lesions, the cell membrane was damaged and the content spilled out. There were a lot of vacuoles in the cytoplasm. And the nuclear was severely deformed with a serrated edge and marginated nuclear chromatin. The microtubules of sucker had partially disintegrated.

Key words: Giardia lamblia, Osthole, In vitro, Ultrastructure