ABSTRACTA new technique was developed to distinguish the four incompatibility interactions inArmillaria mellea sensu stricto.The technique involves confronting two single-basidiospore isolates on oak wood extract agar and then transplanting a strip from the colonies including the interaction zone onto squeezed orange juice agar. A compatible (A≠B≠) interaction results in a change of the colony morphology along the entire agar strip. Morphological characteristics of compatible interactions are highly variable. They may be expressed as appressed or crustose secondary mycelium. In a common B (A≠B=) interaction, such a change occurs only in the interaction zone. The ends of the agar strip remain white and fluffy. Both the common A (A=B≠) and common AB (A=B=) interactions remain fluffy and unchanged. The genotype can be deduced after identifying the A≠B≠and A≠B= interactions. In addition, pairing mycelium produced by the confrontations with known monosporous tester isolates showed that nuclear migration occurs in A=B≠and A≠B≠interactions but does not occur in A≠B= interactions.