Abstract In the fir forest of Mount Parnis 67.67% of the overstorey trees were found infected by the mistletoe (Viscum album). The “6-class” rating system was used to quantify the degree of tree infection. The most intensive mistletoe infection was observed on old age trees. Extensive tree mortality was observed during the years 2000–2002. Low precipitation in two consecutive years (2000–2001) was an important parameter that affected tree mortality. Fir trees were attacked by the bark beetles Phaenops knoteki (Buprestidae) and Pityokteines spinidens (Scolytidae). Mistletoe is an important factor that reduces tree vigour and predisposes trees to beetle attacks during periods of low precipitation. Tree mortality was correlated with the degree of mistletoe infection. During a 5-year-period (1998–2002), mortality of trees with heavy and medium mistletoe infection was 44.44%, while mortality of trees with light infection was 9.5% and that of the non-infected trees 4.66%.