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Comprehensive field data on polystomatid monogeneans record IONA, prevalence and intensity of infection and suggest that worm burdens in this group are strongly regulated: thus, in the majority of Polystoma species infecting anuran amphibians mean abundance is typically less than one parasite/host. There is circumstantial evidence that the dominant control is attributable to host factors which over-ride variations in transmission success. This review provides a brief summary of information on Pseudodiplorchis americanus, a parasite of the desert toad, Scaphiopus couchii, and then focuses in detail on the spectrum of factors regulating infrapopulations of Protopoystoma xenopodis, a parasite of the aquatic Xenopus laevis. Infection levels of adult worms and their contribution to transmission are regulated by external environmental factors (especially temperature), by host factors (including behaviour and population density), and by a range of parasite factors including intra- and inter-specific competitive interactions and variations in intrinsic characters, especially survivorship and reproductive output. In addition to these factors whose primary effect is to modulate transmission rates, there is a major attrition in parasite numbers between invasion and maturity (3 months post-infection). Long-term laboratory experiments on the Xenopus laevis/Protopolystoma xenopodis interaction demonstrate a powerful acquired immune response. Primary infection is characterised by a high prevalence of established adult worms but the success of subsequent challenge infection is greatly reduced, leading to low prevalence and extended pre-patent period. In the small proportion of hosts supporting a second infection of adult parasites, surviving burdens are small (one to two worms/host) and show reduced egg production. These results provide an explanation for the low burdens encountered in field studies: a majority of adult X. laevis in natural populations are likely to exhibit strong, relatively long-term, post-infection immunity after the loss of a previous infection. (C) 2002 Published by Elsevier Science Ltd. on behalf of Australian Society for Parasitology Inc.