The incidence of infection and reproductive success of Phoradendron californicum (desert mistletoe) on its host, Acacia gregii (catclaw), was quantitatively investigated in two localities of the Las Vegas Valley, Nevada, from March through November 1997. Similar patterns were detected between the two study sites. The abundance of Phoradendron was significantly positively correlated with age of Acacia trees, and more positively correlated with size. Phoradendron-infested trees were significantly older and larger, and had significantly lower diurnal and seasonal water potentials than adjacent uninfested trees. Phoradendron-infested trees were significantly more water-stressed than uninfested trees of similar age and size. Infested Acacia trees occurring on an extensive terrace surface were significantly more water-stressed than Acacia occurring along an intermittent wash during the dry summer season. Phoradendron canopy cover and fruit production were significantly larger on Acacia trees occurring along the wash than on the terrace. High levels of Phoradendron infestation greatly increased host-plant water stress, and low host-plant water potentials significantly reduced the canopy cover and fruit production of Phoradendron. However, characteristics of Phoradendron fruits were not significantly different between the two geomorphic surfaces. A combination of age, size and water status of Acacia trees largely influences the infection and reproductive success of Phoradendron in the Las Vegas Valley of southern Nevada.