ABSTRACTClamp connections were observed in basidiospore dilutions from each of four North American biological species ofArmillaria(A. gallica, A. sinapina, A. ostoyae, andA. calvescens) following spore germination and hyphal anastomosis. The occurrence of clamp connections in intrabasidioma pairings of single-germinated basidiospores and of excised margin-hyphae ofArmillaria ostoyaeandA. gallicawas that expected of a bifactorial incompatibility system. All intraspecific pairings between basidiospores or excised margin-hyphae from actively growing haploid cultures from two different basidiomata ofA. ostoyaeand ofA. gallicaproduced clamp connections. The development of the crustose colony morphology was almost fully coincident with the formation of clamp connections in compatible matings with eitherA. ostoyaeorA. gallica.Results indicate that clamp connections occur predictably and provide an additional precise and useful criterion for recognizing biological species ofArmillaria.For unexplained reasons, haploid isolates paired more than 3 mm apart failed to produce clamp connections.