Abstract: The prevalence, mean intensity, relative density, abundance and dispersion of the metacercariae of Centrocestus formosanus (Trematoda: Heterophyidae) was studied on gills of grass carps Ctenopharyngodon idellus from August 1993 to June 1994. The prevalence varied from 62.1% to 100%, and the mean intensity 8.7 to 22.9, the relative density 6.7 to 21.4. The prevalence and mean intensity, relative density were not obviously correlated with the seasonal change of water temperature.The distribution of the metacercariae on the gills was overdispersed. The degree of aggregation increased with the increasing mean density of the cyst infrapopulation The distribution of the metacercarial cysts on the gills showed regular monthly changes with different gill lamellae.