The ecological success of social insects, including ants, is tightly connected with their ability to protect themselves and their food resources. In exchange for energy-rich honeydew, ants protect myrmecophilous aphids from various natural enemies. Fungal infection can have disastrous consequences for both mutualist partners, wherein aphids can be disease vectors. Behavioural responses towards fungus-infected aphids of ant species in nature have scarcely been studied. Here, we studied the behaviour of honeydew foragers of four ant species - Formica polyctena Foerster, Formica rufa L., Formica pratensis Retzius (Hymenoptera: Formicidae, Formicini), and Lasius niger (L.) (Formicidae, Lasiini) - towards Symydobius oblongus (von Heyden) aphids contaminated with the generalist fungal pathogen Beauveria bassiana (Balsamo-Crivelli) Vuillemin in the field. Aphid milkers from Formica spp. quickly detected and removed infected aphids from the host plant (Betula pendula Roth., Betulaceae). Neither ant species, the degree of aphid-milker specialization (medium or high), nor the number of honeydew foragers had significant effects on the behaviour of Formica milkers towards infected aphids. Unlike Formica ants, L.niger usually displayed non-aggressive behaviour (tolerance, antennation, honeydew collection, grooming). By the immediate removal of infected insects, Formica ants seem to minimize the probability of infection of symbionts as well as themselves. Quarantining behaviour may play an important role in ant-aphid interactions as a preventive antifungal mechanism formed under parasite pressure and thus contributing to the ecological success of ants.