Light saturated rates of photosynthesis (Asat), water use-efficiency (WUE) and photosynthetic nitrogen use-efficiency (PNUE) were measured for three sub-Arctic annual root hemiparasites (Rhinanthus minor L., Euphrasia frigida L. and Melampyrum sylvaticum L.) growing in the Torneträsk region of Swedish Lapland. Photosynthesis and resource use-efficiency of the hemiparasites were dependent on (i) attachment to a host and (ii) the nature of the host to which attachment was made. The highest rates of Asat were recorded in those hemiparasites attached to legumes and the lowest rates were recorded in those which were not attached to a host. Intermediate rates of Asat were recorded in hemiparasites attached to grasses. Asat and leaf conductance were more closely related in attached than in unattached hemiparasites, WUE was greatest in those hemiparasites attached to legumes, but was not a simple function of nitrogen supply. PNUE was greater in attached than in unattached plants, but was inversely related to the total-nitrogen concentration of the leaves. In an earlier paper we demonstrated that the growth of these hemiparasites is highly host dependent. Here we demonstrate that the carbon, nitrogen and water relations of these plants are also highly host dependent.