Giardia muris cysts were examined by TEM before treatment, post-induction, and at timed intervals during incubation. Untreated G. muris cysts had a thick cyst wall composed of a fibrous outer wall and a thin, electron dense inner membrane which extended from the trophozoite plasma membrane. The cytoplasm was devoid of endoplasmic reticulum, Golgi bodies, and mitochondria. Numerous large vacuoles were present within the ectoplasm just beneath the plasma membrane in untreated cyst. Following induction these cysts lacked ectoplasmic vacuoles. Concurrently, numerous membrane-bound vesicles were seen in the peritrophic space closely adhering to the surface of the trophozoite. These vesicles appear to be of cytoplasmic origin. The cytoplasm of fully excysted trophozoites lacked ectoplasmic vacuoles but displayed well developed ribbons of microtubules, probably precursors of the ventral disk, lateral flange, and median bodies and also contained extensive endoplasmic reticulum. No more than two nuclei were observed within each organism. The earliest excysted organisms were observed 0-5 minutes after incubation had begun, and most organisms had excysted within 10 minutes. Cytokinesis occurred only after excystation was complete.