Populations of the introduced annual root hemiparasite, Parentucellia viscosa (L.) Caruel (Scrophulariaceae), were examined in habitats where introduced, mixed introduced plus native, or solely native species provided potential hosts. Presence of haustoria on host roots confirmed parasitism of 17 introduced and ten native taxa across the sites investigated. Paired plots, one with all hosts removed early in the season, the other left intact, showed overall increases in shoot dry matter of the parasites over 3 months of growth. Results indicated a substantial growth benefit to the parasite from continued access to introduced and indigenous plant species, although the parasite continued growing to a limited extent after removal of hosts. Carbon isotope discrimination values (delta(13)C) were consistently more negative for parasites than for associated hosts, and more negative for parasites in plots containing hosts than in plots from which hosts had been removed. Values for delta(15)N discrimination of shoot dry matter of parasites lay mostly within the range of those of hosts. The delta(13)C data suggested poorer water use efficiency of parasite than hosts, while corresponding delta(15)N data indicated particularly effective uptake of N-containing compounds from the N-2-fixing host, Lotus angustissimus. Where the parasite had access to only one or a few closely related hosts, the composition of the soluble amino fraction of its leaves generally matched that of foliage of host(s). However, in some instances the parasite accumulated certain amino compounds to a much greater or lesser relative extent than did its partner host(s). Where many hosts were being exploited, compositional features of the soluble amino fraction of the parasite suggested major access to nitrogenous solutes from specific hosts. Data are discussed in relation to previously published observations on haustorial-derived benefits indicated for other species of root hemiparasites. (C) 2000 Annals of Botany Company.