The environmental factors that act on fish parasites have received increasing attention in recent years. The effects of changing environmental conditions on parasite communities can provide valuable information on the effects of pollution on the ecosystem as a whole. This, of course, demands that we have a detailed knowledge o f parasite communities and the mechanisms acting upon them in a variety of natural conditions. Here, Mari Koskivaaro looks at those environmental factors that affect monogeneans parasitic on freshwater fishes.