In Ireland, upland areas are becoming extensively planted with coniferous forest to which sheep are allowed access. Such forest produces many more fruiting bodies of basidiomycetes than the blanket bog on which it was planted. Faecal samples taken from hill and adjacent forest (Picea abies) grazing areas showed an autumnal increase in radiocaesium in the forest samples compared with the hill samples, and there was an indication of higher in vivo radiocaesium activity in the autumn, particularly in individual animals which are known to graze the forest. The peak faecal and in vivo radiocaesium levels coincided with the fungal growing season. A new method is presented using fungal spores, which proves the ingestion of fungi by sheep and permits the identification and quantification of the fungi consumed.