Trichoderma spp. vary in their inhibitory effects towards other fungi (Hadar et al. 1979). The isolate of T. harzianum investigated here demonstrated antagonism towards L. laccata, resulting in overgrowing of the ECM fungal colony where its mycelium became more dense and pigmented. SEM micrographs also indicated some wrinkled or collapsed mantle hyphae of L. laccata on pine roots. These reactions did not appear when the mycorrhizal fungus was challenged with T. virens (Werner at al. 2002) or with M. hiemalis (Werner and Zadworny 2003). Induced pigment production in co-cultured fungi was described as a result of nutritional stress imposed by a more aggressive organism (Calistru et al. 1997; Murphy and Mitchell 2001), while collapse and loss of turgor of hyphae may suggest the influence of extracellular enzymes and/or volatiles produced by the aggressor (Calistru et al. 1997). Therefore, either nutritional stress or extracellular metabolites of T. harzianum could have caused the observed modification in L. laccata. Bursting of T. harzianum hyphae followed by protoplast release and their penetration may suggest an action of cell wall degrading enzymes. Although the parasitic ability of a number of isolates of Trichoderma spp. has been shown (Benhamou and Chet 1993; Elad et al. 1983; Doi et al. 1994), the present SEM investigation failed to indicate any parasitic activity of T. harzianum to L. laccata.