The Argentine ant (Linepithema humile) is a widespread and abundant introduced species that disrupts ecosystems throughout its introduced range. This invader was inadvertently introduced to Santa Catalina, San Clemente, Santa Cruz, and San Nicolas Islands at various points during the past century but currently appears to be absent from the remaining Channel Islands. Multiple spatially disjunct infestations on each invaded island individually range in size from <500 m2 to >427 ha and encompass a variety of habitats, including large areas that are entirely dominated by native perennial vegetation. The existence of multiple infestations on individual islands suggests that inadvertent introduction by humans serves as an important within-island dispersal mechanism. Multiyear surveys of individual infestations on San Clemente Island and Santa Cruz Island reveal approximately radial patterns of expansion (as a result of colony budding) away from the edge of each infestation. Rates of spread by budding on San Clemente Island range from 10 m/year to 57 m/year and are comparable to those on mainland California. Given the documented effects of Argentine ant invasions on Santa Cruz Island, the continued spread of the Argentine ant on the Channel Islands represents a serious environmental concern. Eradication programs underway on San Clemente and Santa Cruz Islands will hopefully result in island-wide elimination of this detrimental invader. Native to southern South America (Wild 2004), the Argentine ant (Linepithema humile) has become an abundant and disruptive invader in many parts of its introduced range (Holway et al. 2002a). The Argentine ant was first found in California as early as 1905 (Smith 1936) and now widely occurs there, especially along the coast and in the Central Valley (Ward 1987, Holway 1995, Suarez et al. 1998). Areas invaded by L. humile in Califor- nia support few native ant species (Tremper 1976, Ward 1987, Human and Gordon 1996, Holway 1998a, 1998b, 2005, Suarez et al. 1998, Mitrovich et al. 2010, Hanna et al. 2015b). Additional impacts of this invasion include reduced prey availability for vertebrate insectivores, such as horned lizards (Suarez and Case 2002) and shrews (Laakkonen et al. 2001), and the disruption of seed dispersal (Bond and Slingsby 1984) and pollination mutualisms (Hanna et al. 2015b).