Invasive species can have large effects on freshwater communities and ecosystems. Potamopyrgus antipodarum, a snail indigenous to New Zealand, has recently colonized North America. We documented the distribution and density of P. antipodarum in the major tributaries of the Madison River in the Greater Yellowstone Ecosystem, examined associations between P. antipodarum and native macroinvertebrates, and experimentally examined how P. antipodarum affected colonization of substrates by other macroinvertebrates. In 1997 and 1998, we sampled benthic macroinvertebrates in the Madison, Firehole, and Gibbon Rivers and Nez Perce Creek in 3 seasons. In 1999, we examined colonization of P. antipodarum and other macroinvertebrates in a field experiment using slate tiles in the Madison River. In September 1997, we found P. antipodarum at 23 of 32 locations. Where they were present, P. antipodarum densities ranged from 22 to 299,000 ind./m². Densities of P. antipodarum declined between September 1997 and March 1998 and increased between March and July 1998. The composition and structure of native macroinvertebrate assemblages covaried with P. antipodarum densities. Whether the differences in macroinvertebrate communities among sites were related to differences in physical characteristics of streams or to interactions with P. antipodarum was unclear. However, in the field experiment, high densities of P. antipodarum were associated with low colonization of other macroinvertebrates; thus, negative interactions between native macroinvertebrates and P. antipodarum may have the potential to influence the large-scale distribution of other macroinvertebrates.