Effect of trichlorfon (0.5 ppm) on the ventilation in the black-lip pearl oyster Pinctada margaritifera was examined. The ventilation volume was measured continuously during the experiment by the direct method. About 43% of the black-lip pearl oysters which were exposed to the trichlorfon solution expelled the dead bodies of the parasite, Conchodytes meleagrinae, out of the mantle cavity and their ventilation volumes did not differ significantly from those of the pearl oysters which did not expel the dead parasites. After exposure to the trichlorfon solution, all the pearl oysters survived one week or more. It is unlikely that both the presence of parasite in the supra-branchial cavity and the exposure to 0.5 ppm trichlorfon solution affect the ventilation volume in the pearl oyster.