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1. Previous studies in humid forests have shown that the C-13 and N-15 isotopic composition differs between ectomycorrhizal (ECM) and saprotrophic (SAP) fungi, and that this ECM-SAP ’divide’ may provide a useful tool for evaluating fungal trophic status.

2. We evaluated whether this method could delineate the trophic status of fungi in two semiarid, temperate forests of the south-western USA. This technique could be particularly valuable in arid regions where the functional roles of fungi can be difficult to assess because of infrequent sporocarp production.

3. Our data were consistent with the existence of an ECM-SAP divide, although delta C-13 values were more useful than delta N-15 values in separating trophic status. Saprotrophic fungi consistently had higher delta C-13 values than their presumed substrates; however, the degree of N-15 enrichment in SAP sporocarps was highly variable. Comparison of 11 sporocarp species common to both sites showed that delta N-15 values were higher in one of the forests, even though the delta N-15 values of foliage from common understorey and overstorey trees were similar between forests.

4. We conclude that assessment of the isotopic compositions of fungal sporocarps and their substrates is helpful for elucidating ecological relationships in semiarid forests.