About 35 Trichoderma species are currently recognised on the basis of morphological and molecular characters. Besides the role of a few of these species in biotechnology, several seem to play prominent roles in soil ecosystems. With a goal of investigating global biodiversity in Trichoderma, we report on the occurrence of Trichoderma spp. in Russia (Moscow and Ural areas), Siberia (Krasnoyarsk area) and the Himalayan mountains – areas from which no Trichoderma isolates are so far available. The ITS 1 and 2 sequence of the rDNA cluster of the 75 isolates obtained was compared with that of ex-type strains and taxonomically established isolates of Trichoderma. Thirty-nine isolates were positively identified as T. atroviride, T. virens, T. hamatum, T. asperellum, T. koningii and T. oblongisporum. A further 26 isolates yielded six closely related ITS1/2 sequence types, which are highly similar yet different from the ex-(neo)type strains of T. harzianum and T. inhamatum. Some of these genotypes (i.e. 1 and 2a) occurred only in Russia/Siberia, whereas others (2b, 3, 4 and 5) were found only in the Himalayas. RAPD analysis was consistent with these genotypes, and revealed genetic homogeneity even between strains from widely separated areas. Parsimony analysis placed these five genotypes, together with T. harzianum, T. inhamatum and the mushroom-aggressive T. harzianum ‘biotype 2’ in a large, unresolved ‘harzianum’ clade. Ten isolates were not safely alignable within known species, and five of them may be undescribed taxa: one isolate from 2700 m elevation in the Himalayas, which clustered in parsimony analysis at a basal position in section Longibrachiatum; and four isolates, displaying two closely related sequence types, forming a separate clade with T. stromaticum. The five remaining isolates also exhibited three unique ITS1 and 2 sequence patterns, but parsimony analysis placed them into the unresolved ‘harzianum’ clade, and their relationship to T. harzianum is thus unclear. The study shows that molecular screening of uninvestigated geographic areas can lead to the identification of isolates with new ITS1 and ITS2 sequence patterns, some of which may be new taxa. It also reveals that T. harzianum is at present the genetically most diverse member of the genus.