Abstract Acute cyanobacterial hepatotoxicosis in a wild roe deer ( Capreolus capreolus) from Norway is reported. The diagnosis was based upon the demonstration of typical liver lesions and high liver concentrations of microcystins. The liver was markedly enlarged and histopathological examination revealed diffuse hepatocellular dissociation, degeneration and necrosis and perisinusoidal haemorrhage. Liquid chromatography–mass spectrometry demonstrated the presence of 1361 ng microcystin-YR, -LR and -RR per gram liver (wet weight). This is believed to be the first report of cyanobacterial intoxication in wild mammalian species as confirmed by demonstration of high toxin levels in the animal’s tissues.