Salt cedar (Tamarix ramossisima), an invasive species, has become a dominant shrub along many streams of the southwestern United States, where it has replaced many native species such as Fremont cottonwood (Populus fremontii). We examined whether the successful invasion of this exotic shrub alters stream leaf litter decomposition rates and affects the aquatic macroinvertebrates that are dependent on leaf litter as a food source. With an in-stream leaf pack experiment, we found that faster decomposition of salt cedar litter was associated with a two-fold decrease in macroinvertebrate richness and a four-fold decrease in overall macroinvertebrate abundance, relative to native Fremont cottonwood. Macroinvertebrate communities were also significa ntly different on the two food sources through time. These studies demonstrate that invasion by salt cedar affects leaf litter quality, which in turn affects stream macroinvertebrates. Such impacts on the primary consumers and food web structure could affect higher trophic levels.