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Tamarix aphylla is an evergreen tree that has invaded the drawdown zone of Lake Mead, a large reservoir on the Lower Colorado River. We performed competition experiments between T. aphylla and T. ramosissima, and between T. aphylla and the native tree Salix gooddingii. Root:shoot ratios and biomass were higher in S. gooddingii than both Tamarix species, and T. ramosissima grew taller than T. aphylla and S. gooddingii when treatments with single plants and no competition were compared. Tamarix aphylla outcompeted the native S. gooddingii, but had competitive abilities that were slightly inferior to T. ramosissima. The competitive abilities of T. aphylla may and help explain why this species is not as widespread as its congeners, although because of T. aphylla’s larger size, the species may be as serious a threat to native riparian ecosystems as T. ramosissima. These results indicate that management actions should be taken to ensure that T. aphylla does not further invade riparian ecosystems in the southwestern United States.