We investigated the occurrence and genetic diversity of Trichoderma in the river Danube national park, a primeval, riparian forest area located south-east of Vienna (Austria) which represents one of the last cases of an original European river-floodplain landscape. Forty-six strains were isolated and identified at the species level by analysis of morphological characters, by sequence analysis of their internal transcribed spacer regions 1 and 2 (ITS 1 and 2) of the rDNA cluster and--in some cases--a fragment of the translation elongation factor 1alpha (tef1) gene, and RAPD-analysis. Twenty-one strains were positively identified as T. harzianum, thirteen as T. rossicum, four as T. cerinum, two as T. hamatum, and one each as T. atroviride and T. koningii: four strains yielded two different ITS1 and 2 as well as tef1 sequence types, which were not alignable with any known species. Our studies show that they represent two new taxa of Trichoderma.