We measured leaf dry matter delta(18)O and delta(13)C in parasitic plants and their hosts growing in southwestern Australia. Parasite/host pairs included two mistletoe species, three species of holoparasites, and five species of root hemiparasites. Among these parasite functional types, significant variation was observed in parasite/host isotopic differences for both delta(18)O ( P<0.0001, n=65) and delta(13)C ( P<0.0001, n=64). Mistletoes were depleted in both (18)O and (13)C compared to their hosts; parasite/host differences were -4.0 per thousand for delta(18)O ( P<0.0001) and -1.9 per thousand for delta(13)C ( P<0.0001). The lower delta(18)O in mistletoe leaf dry matter compared to their hosts is consistent with the frequently observed high transpiration rates of these parasites. Root hemiparasites were also depleted in (18)O and (13)C compared to their hosts, but not to the same extent as mistletoes; parasite/host differences were -1.0 per thousand for delta(18)O ( P=0.04) and -1.2 per thousand for delta(13)C ( P=0.0006). In contrast to mistletoes and root hemiparasites, holoparasites were enriched in both (18)O and (13)C compared to their hosts; parasite/host differences were +3.0 per thousand for delta(18)O ( P<0.0001) and +1.5 per thousand for delta(13)C ( P=0.02). The enrichment in (18)O for holoparasite dry matter did not result from more enriched tissue water; holoparasite tissue water delta(18)O was less than host leaf water delta(18)O by a difference of -3.8 per thousand when sampled at midday ( P=0.0003). Enrichment of holoparasites in (13)C compared to their hosts is consistent with a generally observed pattern of enrichment in heterotrophic plant tissues. Results provide insights into the ecology of parasitic plants in southwestern Australia; additionally, they provide a context for the formulation of specific hypotheses aimed at elucidating mechanisms underlying isotopic variations among plants.