The objective of this study was to measure levels of the toxin microcystin in different tissues of fish known to feed on cyanobacteria during toxic bloom events. Wild Nile and redbreast tilapia (Oreochromis niloticus and Tilapia rendalli) were sampled from the catch of artisanal fishermen at eutrophic stations of Funil and Furnas reservoirs in southeastern Brazil. Phytoplankton communities in the two reservoirs were quite different taxonomically, but not dissimilar in microcystin content (200 microg g dry weight (DW) seston(-1) at Funil, 800 microg gDW seston(-1) at Furnas). All of the 27 fish sampled contained microcystin, ranging from 0.8 to 32.1 microg g liver(-1) and from 0.9 to 12.0 ng g muscle(-1). Most microcystin variants found in seston were also found in fish liver. T. rendalli had the lowest concentration in both tissues when compared to O. niloticus. In both reservoirs, one of every four fish sampled, always O. niloticus, had a level of microcystins beyond the World Health Organization tolerable daily intake (8 ng g tissue(-1)) and represented a risk for consumers. It is possible that closer study of inter-species variability in toxin burden in cyanobacteria-impacted water bodies will permit the development of guidelines for fish consumption that will better protect public health.