During its invasion of North America, the exotic Daphnia lumholtzi has rapidly colonized numerous reservoirs, natural lakes, and large rivers. In the current study, we examine the overlap between D. lumholtzi and native Daphnia species through analysis of two data sets: co-occurrence in reservoirs of the south-central United States and population dynamics in one reservoir. In 171 reservoirs and oxbow lakes, D. lumholtzi was among the most prevalent species and its distribution was independent of other Daphnia species. Over a 28-month period in Stockton Lake, Missouri, D. lumholtzi was abundant only in late summer, a period when the epilimnion was warm (25-30??C) and cyanobacteria were common. Native Daphnia (D. mendotae, D. parvula, and D. retrocurva) complemented this pattern, being generally rare in summer and most abundant during winter and spring. Peak densities and average fecundities of native Daphnia during the cooler months were typically greater than densities and fecundities of D. lumholtzi in summer. The complementary population dynamics between native and exotic species may be significant for the food webs in warm reservoirs, with D. lumholtzi providing another food resource when the abundance of large zooplankton is ordinarily low. ?? 2006 E. Schweizerbart'sche Verlagsbuchhandlung.