There is a lack of documentation on the consumption of toxic fruits by carnivores. To quantify the importance of toxic fruits in the diets of badgers (Meles meles), foxes (Vulpes vulpes) and martens (Martes sp.), we collected faeces of these animals along nine transects (total 30.4 km long, 2 m wide) during the fruiting seasons (June–November) in 2010 and 2011. From 619 faecal samples we found seeds of 16 fleshy-fruited plant species, including four species with toxic fruits. Surprisingly, the fruit of Convallaria majalis having a high content of toxic substances was also eaten by these animals. Fruits of C. majalis were found in faeces in late summer/autumn after early frost when maturing processes had reduced the content of toxic substances in the mesocarp. Toxic fruits were detected in 6.3% of all faeces sampled (n = 619). The proportion of toxic fruits in the diet was the highest (up to 10.5%) in martens as compared with that in badgers and foxes (2.2% and 0.7%, respectively).