Recent studies documented the potential transfer of microcystin, a hepatotoxin produced by some cyanobacteria, from aquatic to terrestrial ecosystems. Using enzyme linked immunosorbent assay, we measured microcystins in emergent Hexagenia limbata mayflies and fecal samples collected from a maternity colony of little brown bats (Myotis lucifugus) located adjacent to a Michigan (U.S.A.) lake that experiences seasonal blooms of toxicogenic Microcystis aeruginosa. All H. limbata and M. lucifugus fecal samples contained microcystin (H. limbata: mean = 293.88 ng/g dw +/- 35.99 SE, n = 39; M. lucifugus: mean = 262.10 ng/g dw +/- 31.08 SE, n = 20). Ingestion of this toxin may represent a previously unrecognized stressor on bat populations in this region.