Burrowing mayflies, genus Hexagenia, were extirpated from the major water bodies of North America in the early 1950s, paralleling an increase in eutrophication and organic pollution, and a decrease in dissolved oxygen concentrations. Burrowing mayflies recolonized the western basin of Lake Erie, but remain absent in other former habitats such as Oneida Lake, New York. Eutrophication is commonly associated with a shift in the phytoplankton community toward dominance by cyanobacteria, and therefore, one class of cyanobacterial toxins, microcystins, were investigated as a contributing factor to Hexagenia's eradication or as an impediment to recolonization. Laboratory experiments were conducted to determine if microcystin-LR (MC-LR) produced negative effects on Hexagenia at three points within its life cycle: egg, hatchling nymph (<24-h old, <1 mm total length), and pre-emergence nymph (>17 mm). Treatment concentrations ranged from the guideline set by the World Health Organization for drinking water (0.001 microg mL(-1)) to 0.1 microg mL(-1) for the egg experiment and 10 microg mL(-1) for the nymph trials. Eggs showed a delay in hatching and an altered distribution of hatching over the study period when submerged in 0.1 microg mL(-1) MC-LR (an elevated concentration representative of bloom scum). The 72-h (1.1 microg mL(-1)) and 96-h (0.049 microg mL(-1)) LC(50) values for hatchling nymphs exceeded typical bloom concentrations of North American lakes, (0.01 microg mL(-1)). Large nymphs were more tolerant of the toxin, as indicated by 100% survival over seven days exposure to 10 microg mL(-1), suggesting older larvae can withstand brief encounters with high microcystin levels for at least short periods of time. The sensitivity of young nymphs and eggs to MC-LR may have implications for the recruitment of the genus in water bodies with persistent summer cyanobacterial blooms.