Competition is likely to play an important role in ectomycorrhizal fungal interactions, but the mechanisms driving competitive outcomes among these fungi are not well understood. Competition may be influenced by interactions with the host plant, fungal-specific traits, or a combination of both factors. In this talk, we summarize a group of studies examining competition among species of ectomycorrhizal fungi in the genus Rhizopogon that associate with Pinus muricata. We show that fungal traits such the timing of spore germination and rhizomorph production can play key roles in determining the composition of fungi found on seedling roots and that competition can also be indirectly mediated through interactions with a shared host. We discuss the role of these mechanisms in the context of ectomycorrhizal assemblage dynamics in natural stands of Pinus muricata at Point Reyes National Seashore.