Heterobasidion annosum causes root and butt-rot in trees and is the most serious forest pathogen in the northern hemisphere. We developed a rapid and simple Agrobacterium-mediated method of gene delivery into H. annosum to be used in functional studies of candidate genes and for visualization of mycelial interactions. Heterobasidion annosum TC 32-1 was cocultivated at pH 5.6 and 20 degrees C in Hagems medium with Agrobacterium tumefaciens C58 carrying plasmids with hygromycin B resistance as the selectable marker and green fluorescent protein as a visual marker. We obtained 18 mitotically stable transformed isolates showing green fluorescence protein activity.