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.