32P was used to determine phosphorus uptake by different mycorrhizas of birch in the field. Time of year and location of the application of 32P were the dominant variables influencing uptake into the canopy. There was evidence of differential uptake by the three mycorrhizal types selected. Where Hebeloma mycorrhizas were more frequent, a significantly greater rate of phosphorus uptake per unit leaf weight occurred than where Laccaria or Lactarius were more frequent. The method shows potential for field evaluation of efficiency of nutrient uptake by mycorrhizas, although a number of factors need to be taken into account in determining uptake. In locating areas for 32P injection, it was found that the distribution of mycorrhizal types on roots did not always reflect the distribution of the corresponding fruitbodies.