Abstract: The present paper deals with the monthly change of metacercarial cyst infrapopulation ofthe digenean, Centrocestusfor mosanus, In the lamellae ofgills ofmosquito fishes (Gambusia affinis).The population number and weight ofmosquito fishes fluctuated similarly in twelve months.The population body structure of mosquito fishes was correlated with the seasonal variation ofwater temperatures.The prevalence, relative density and mean intensity of infection varied respectively 89192 ? 7103%, 5134 ?1193, 4190 ? 2101, and were only directly correleted with the body length ofthehost mosquito fishes,not with the seasonal varia2tions ofwater temperature.The longer the body length,the higher the prevalence, and vice versa, and so were the mean intensityand relative density.The distribution ofthe metaceriae cysts in the gills showed similarmonthly regular changes in different groupsofgill but the metacercariae cysts number of group 2 and group 3 were more than group 1 and group 4.The distribution of themetacercariae cysts in the gills ofmosquito fishes sampled in different months showed aggregated distribution with a large numberof cysts being in a small number of mosquito fishes, and most mosquito fishes were infected with less cysts, but the degree ofag gregation increased with the mean density ofthe metacercarial cyst infrapopulation. From the spread way and effect and the samehabitat of the mosquito fish and the definitive host of this parasite,We can infer that the mosquito fish was one of kind ofthemostadaptive and stable second intermediate host of Centrocestusformosanus.