Pleurotus ostreatus 'Miso' is a mutant strain showing white color in pileus from the known parent strain 'Wonhyeong No. 1'. Shape and several other characters also vary with culture conditions. Mating experiments were performed to understand interstrain mating relationship using monokaryons of the parent and the mutant strains. All monokaryons were grown from single spores isolated from freshly collected fruit bodies. Pairings were performed in 90 mm petri dishes on PDA. They were allowed to grow at 25℃ until two fronts of the advancing mycelia met and developed a conspicuous contact zone. The contact zone and the outer edges of paired colonies on each plate were examined for clamp connections. The crosses were considered positive when clamp connections were at the growing margin of either side of the interacting monokaryons. The parent strain 'Wonhyeong No. 1' and the mutant strain 'Miso' resulted in tetrapolar incompatibility system in intrastrain crosses. Normal compatible matings could be visibly distinguished from incompatible ones by the rapid growth and gross morphology. The resultant dikaryotic colony was also confirmed microscopically by the presence of clamp connections from incompatible pairings. In interstrain crosses, each monokaryotic tester strains of the parent strain was out-crossed to monokaryotic tester strains of the mutant. The four classes of tester strains representing the four incompatibility types were chosen from each mutant and parent strains. As a result of these crosses it was found that both strains share the same A and B incompatibility factors yielding 25% compatibility.