The impact of soil salinity and host water status on the host / parasite association between Eucalyptus largiflorens (F.Muell.) and Amyema miquelii (Lehm. ex Miq.) Tiegh. was investigated in a semi-arid floodplain environment in southern Australia. Water status of potential hosts (i.e. uninfected E. largiflorens) was assessed at a range of sites with different soil salinities and then compared with mistletoe infection at the same sites. Pre-dawn leaf water potentials (Ψ) of uninfected E. largiflorens declined with increasing salinity, while leaf δ¹³C values increased. The proportion of infected E. largiflorens at each site decreased significantly with increasing soil salinity. A significant relationship was found between the proportion of infected trees at each site and leaf δ¹³C values, but not pre-dawn Ψ, of potential hosts. The impact of mistletoes on water status of infected trees was also investigated. Among infected trees, we found no significant relationship between mistletoe volume and either pre-dawn Ψ or leaf δ¹³C values of hosts. However, there was a significant relationship between host midday leaf Ψ and mistletoe volume, with hosts exhibiting increased stress as mistletoe volume increased. The data suggest that increasing water and / or salinity stress make E. largiflorens a less suitable host for A. miquelii.