Abstract: The population distribution and seasonal alternation of Dactylogyrus hypophthalmichthys on gills of silver carp Hypophthalmichthys molitrix, and D.lamellatus on grass carp Ctemopharyngodon idellus were studied.Two species of fish were collected in a fish pond in the Institute of Hydrobiology,Chinese Academy of Sciences in Wuhan,China from December 1995 to November 1996.The population dynamics of abundance and prevalence were similar for the two species of monogeneans.The prevalence was higher than 70% for D.hypophthalmichthys and higher than 60% for D.lamellatus, and no significant difference(P<0.05)was found between months for the two species of monogeneans.The abundance of D.hypophthalmichthys increased gradually from February to March,and reached the highest level in April,then declined gradually and remained at a low level in autumn and winter;while D.lamellatus increased from November to March,and reached the highest value in April,then decreased rapidly and maintained a low level in summer and autumn.Statistical analysis indicated that changes in abundance of D.hypophthalmichthys and D.lamellatus were both significantly seasonal (P<0.05).The pattern of distribution of D.hypophthalmichthys was over-dispersed,as the variance to mean ratio was more than 1 and the degree of aggregation was high in spring and autumn.The pattern of D.lamellatus was also over-dispersed,with its the degree of aggregation being high in spring and winter.