Background. The extent to which ectomycorrhizal fungi mediate primary production, carbon storage, and nutrient remineralization in terrestrial ecosystems depends upon fungal community composition. However, the factors that govern community composition at the root system scale are not well understood. Here, we explore a potential tradeoff between ectomycorrhizal fungal competitive ability and enzymatic function. Methods. We grew Pinus muricata (Bishop Pine) seedlings in association with ectomycorrhizal fungi from three different genera in a fully factorial experimental design. We measured seedling growth responses, ectomycorrhizal abundance, and the root tip activity of five different extracellular enzymes involved in the mobilization of carbon and phosphorus. Results. We found an inverse relationship between competitiveness, quantified based on relative colonization levels, and enzymatic activity. Specifically, Thelephora terrestris, the dominant fungus, had the lowest enzyme activity levels, while Suillus pungens, the least dominant fungus, had the highest. Discussion. Our results identify a tradeoff between competition and function in ectomycorrhizal fungi, perhaps mediated by the competing energetic demands associated with competitive interactions and enzymatic production. These data suggest that mechanisms such as active partner maintenance by host trees may be important to maintaining "high-quality" ectomycorrhizal fungal partners in natural systems.