In northeastern California oak woodland, we measured nitrogen uptake by ectomycorrhizal (ECM) oak roots (Quercus douglasii, blue oak, Q. wislizeni, live oak). In February 2005, after sampling fine roots of 5 blue and 5 live oaks, we added ¹⁵NH₄ to soils, waited one month then re-sampled. ECM root tips were separated into 21 morphotypes and analyzed for ¹⁵N. Blue oak ECM diversity was higher than that in live oak. Few morphotypes (33%) were common to both oak species. More abundant morphotypes took up less ¹⁵N than less abundant morphotypes. We hypothesized that ECM species more active in N uptake would be more frequent in soil cores. However, our data did not support our hypothesis; and ECM species present in few cores were more enriched in ¹⁵N than species present in many soil cores. We believe our data are the first to demonstrate functional biodiversity of ECM fungi for N uptake.