This study documents the distribution of the Argentine ant (Linepithema humile), a well known invader of urban and agricultural ecosystems, in a riparian corridor near San Luis Obispo, California. Sampling of the ant community was done using pitfall traps placed along transects that crossed three vegetation types found at the study site: (1) riparian woodland dominated by coast live oak (Quercus agrifolia), poison oak (Toxicodendron diversilobum), and willow (Salix spp.) (2) Hard chaparral dominated by scrub oak (Q. dumosa) and silk-tassel bush (Garrya sp.) (3) Coastal scrub dominated by black sage (Salvia mellifera), California sagebrush (Artemisia californica), and Yucca (Yucca whipplei). Linepithema humile has become well established throughout the lower reaches of the study site except in the chaparral vegetation. Ten species of native ants were found in the study area, but there was little to no correlation or cooccurrence between L. humile and native ant species. Total species diversity in areas where L. humile occurred was significantly less than in areas where L. humile was absent. These results suggest that invasion by L. humile is restricted by habitat type, and has significant negative impacts on native ant diversity.