Tree of heaven (Ailanthus altissima) is an oriental tree listed as an invasive alien species of Union concern. Its establishment and spread may be especially harmful in protected and ecologically sensitive environments. In 2021, a survey was carried out in Krka National Park to determine the presence of fungi on tree of heaven plants showing a decline, dieback or wilt symptoms. Eighteen symptomatic A. altissima plants were found in five locations within Krka National Park. These symptomatic trees were sampled and analysed for the presence of fungi. Fungi isolated were identified by morphology and ITS1/ITS4 sequencing. Fifteen fungal species were identified: Diaporthe chamaeropis, D. eres, D. foeniculina, D. ravennica, Diplodia mutila, D. seriata, Dothiorella viticola, Fomitiporia mediterranea, Fusarium oxysporum, F. solani, Paraconiothyrium brasiliense, Peroneutypa scoparia, Rosellinia corticium, Schizophyllum commune and Verticillium dahliae. The isolated fungi range in their ecological niches from secondary colonisers of the declining tree of heaven wood, to possible endophytes, and plant pathogens.