toxic cyanobacteria are common in portuguese freshwaters and the most common toxins are microcystins. the occurrence of microcystin-lr (mcyst-lr) has been reported since 1990 and a significant number of water reservoirs that are used for drinking water attain high levels of this toxin. aquatic animals that live in eutrophic freshwater ecosystems may be killed by microcystins but in many cases the toxicity is sublethal and so the animals can survive long enough to accumulate the toxins and transfer them along the food chain. among these, edible mollusks, fish and crayfish are especially important because they are harvested and sold for human consumption. mussels that live in estuarine waters and rivers where toxic blooms occur may accumulate toxins without many significant acute toxic effects. in this study data are presented in order to understand the dynamics of the accumulation and depuration of mcyst-lr in mussels. the toxin is readily accumulated and persists in the shellfish for several days after contact. in the crayfish the toxin is accumulated mainly in the gut but is also cleared very slowly. in carps, although the levels of the toxins found in naturally caught specimens were not very high, some toxin was found in the muscle and not only in the viscera. this raises the problem of the toxin accumulation by fish and possible transfer through the food chain. the data gathered from these experiments and from naturally caught specimens are analyzed in terms of risk for human consumption. the occurrence of microcystins in tap water and the incidence of toxic cyanobacteria in fresh water beaches in portugal are reported. the portuguese national monitoring program of cyanobacteria is mentioned and its implications are discussed.