Nine species of basidiomycota and one species of ascomycota were grown in an ammonium sulphate media and on beech wood; and the general N-15 dynamic patterns of the hyphae were examined. The fungal body initially became depleted in N-15 in both the types of incubation. However, the underlying mechanisms were quite different, that is, significant fungal N-15 drop on the beech wood is associated with the fungal N reallocation and the uptake of atmospheric ammonia and/or NOx, in addition to isotope fractionation during assimilation. Although the N-15 values of the wood-decomposing basidiocarps were generally close to the N-15 values of the wood, it does not always indicate that the wood derived N was the sole N source for the fungi throughout the growth periods as shown in our wood-decomposing experiment.