Sequence-based markers were developed to study the genetic structure and reproductive biology of the ectomycorrhizal fungus Suillus pungens Thiers & Smith in a Bishop pine (Pinus muricata D. Don) forest. Six different basidiome genotypes were found in a 1200 m-2 area. Five of the six genotypes were represented by single basidiomes. The remaining genotype comprised 13 basidiomes and covered an area of at least 300 ml, with maximum measured dimensions of 40 m and 14 m. This is the largest genet of an ectomycorrhizal fungus described to date, and is likely the result of vegetative growth, because analysis of single spore isolates eliminates the possibility of genetic identity resulting from either apomixis or fortuitously indistinguishable recombinant genotypes. Genetic analysis also shows that although out-crossing appears to predominate in the population, at least a low percentage (1.4%) of spores are secondarily homothallic. The combination of extensive vegetative growth and abundant fruiting suggests S. pungens utilizes more carbon than might be expected for a species which accounts for lt 3% of the total ectomycorrhizal abundance at the site. Additional carbon might come from either more efficient host-fungus transfer, pooling of carbon from the roots of different host plants, or saprophytism.