Abstract: Trophozoites of Giardia lamblia were axenically cultivated with modified TYI-S-33 medium contained 500 μg/ml metronidazole（12h LC₅₀）. The morphology of drug-treated trophozoites was observed with light and electron micro-scopes at 2, 4, 8, 12 h respectively. The light microscopy revealed that the trophozoites treated with MTZ showed swollen, detached from the wall of the culture tube, and were with vacuoles in the cytoplasm. Movement of the flagella become slowly or stopped. Electronic microscopy showed that the trophozoites were swollen and deformed; lots of vacuoles were seen in the cytoplasm; the contents of cytoplasm were depleted and the nuclei deformed. This study indicated that MTZ has injured the morphology of G.lamblia.

Key words: Giardia lamblia, Motronidazole, Morphology