Abstract: 8 hours after pyquiton treatment at a single dose of 100mg/kg, the cytoplasmic projections of the teguments of P. westermani were shortened, their secretory bodies being decreased. Contraction and swelling of the worms were seen 16-24 hours after treatment. Various organelles of the teguments including syncytia, muscle, genital organs, and the flame cells showed irreversible damage, especially on the mitochondria (Figs. 1-7). The activities of the SDH, ACP and ANAE were gradually reduced and finally disappeared.. The dead worms disintegrated after 3-15 days.