The extraradical mycelium of ectomycorrhizas is comprised of a network of hyphae that may initiate rhizomorphs, sclerotia and sexual reproductive structures. The development of these structures requires photosynthates produced by host trees. In this study, the initiation and early development of Laccaria bicolor (Maire) Orton fruitbodies (basidiocarps) were studied. Seedlings of Pinus resinosa Ait. and Pinus sylvestris L. were colonized by L. bicolor, a broad host epigeous ectomycorrhizal basidiomycetous fungal species, in growth pouches. Ectomycorrhizas with an extensive extraradical mycelium formed on short roots within 7-12 days after fungal inoculum was introduced. Numerous sites of hyphal aggregation, many of which subsequently developed into basidiocarp primordia, were initiated in the extraradical mycelium. Initial changes in aggregating hyphae included swelling and branching followed by growth of hyphae perpendicular to the paper wick in growth pouches. A stipe and a pileus primordium developed but none of these basidiocarp primordia matured. Distinct regions were evident in the stipe and pileus when sections were stained by various methods. Loose hyphae at the apex and periphery of the pileus were separated by mucilage. In Petri dishes with vermiculite as the substrate, basidiocarps of Laccaria laccata formed in ectomycorrhizal associations with Pinus sylvestris, developed a mature pileus with an hymenium and mature basidiospores.