Development of rhizomorphic thalli of Armillaria mellea is described from observations of petri-plate cultures of many isolates from several sources. Thallus development is divided into 5 stages: pre-emergence of rhi-zomorphs, emergence, lag phase of rhizomorph growth, linear, and terminal phases. Some details of growth and anatomy are described and illustrated. Experiments are reported indicating that tha mycelia must be exposed to the atmosphere directly in order to initiate rhizor-morphs. The rhizomorphs of this species easy to grow in culture for either experimental or classroom use.