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The population biology of Paradistomum orientalis, a dicrocoeliid trematode occurring in the gall bladder of the garden lizard Calotes versicolor, was investigated during 1991±1993. The prevalence and abundance of the¯uke were high throughout the year and demonstrated no seasonal¯uctuations. The maturation cycle of the¯uke followed an annual pattern. The recruitment of the¯uke was restricted to the period September to February, with a peak during September. Mature¯ukes occurred throughout the year, facilitating continuous release of eggs into the macroenvironment. Rainfall plays a major role in governing the maturation cycle, by regulating the population density of the snail host and the feeding activity of the de®nitive host. The frequency distribution of the¯uke followed the overdispersion pattern, ®tting the negative binomial model. There is no evidence for the existence of parasite-induced host mortality. On the other hand, competition for space and nutritive resources under crowded conditions serves as a major regulatory force and is the key factor responsible for maintaining the equilibrium of parasite densities during different months. The infrapopulation of the¯uke remained stable over several years.

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