The population biology of metacercariae of Centrocestus formosanus (Nishigori, 1924) on the gills of the larvivorous fish, Aplocheilus panchax (Hamilton) was investigated based on data from examination of monthly samples of fish collected from a stream at Visakhapatnam during the period December 1983 to April 1986. The seasonal dynamics, dispersion pattern and long-term changes were studied. No seasonality in prevalence and density of infection with metacercariae was observed. The recruitment of metacercariae occurred throughout the year. An increase in density of infection with increase in size of the fish was noted. Possible reasons for the observed distribution are discussed. The frequency distribution of the metacercariae followed the negative binomial at different times of the year. The data indicated an increase in degree of overdispersion with increase in size offish except for a slight drop in the largest size class. Long-term changes in the abundance of metacercariae are given and a decline in parasite population abundance is noted.