In our study also, differences in effectiveness between fungi could not always be related to colonized root length. Jones et al. (1990) attributed differences in the effectiveness of two ectomycorrhizal fungi which colonized the same length of root, to differences in the develop- ment of hyphae in soil by these fungi. We related the growth of plants to hyphal development in soil, but found that the most effective fungus in increasing plant growth (D. maculata) formed the least amount of external hyphae per m of colonized root. Fur- thermore, isolates of L. laccata developed more external hyphae per m of root than other fungal isolates without any apparent additional benefit to the plant. Hyphal development in soil therefore appeared to be a poor indicator of mycorrhizal effectiveness. The spread of hyphae in soil (Jakobsen, Abbott & Robson, 1992) and the proportion of metabolically active hyphae in soil (Hamel, Fyles & Smith, 1990) may be better indicators of mycorrhizal effectiveness than total hyphal length (measured in our study), and these possibilities need to be tested. Other functional differences between ectomycor- rhizal fungi may also affect their ability to increase plant growth. For example, plants inoculated with isolates of P. tinctorius retained a greater proportion of P in their roots than plants inoculated with other isolates. This storage of P may be an important attribute in environments where the growth period is seasonal or intermittent. Further work is required to identify characteristics of ectomycorrhizal fungi which can be used in the screening and selection of inoculant fungi