1. Although large cladocerans are usually uncommon in large rivers, Daphnia lumholtzi (an exotic species in North America) is widespread and occasionally abundant. We collected zooplankton on the Illinois River (Illinois, U.S.A.) in 1995 and 1996 and found that the peak density of D. lumholtzi (25 L-1) typically exceeded that of all native species combined. Maximum density occurred during warm periods (up to 27 degrees C) when concentrations of inorganic suspended sediments were high ( 50 mg L-1). 2. Using a life table experiment, we examined the effects of variation in suspended sediment (0 and 80 mg L-1) and food (10(4) and 10(5)Ankistrodesmus cells mL(-1)) on fitness of D. lumholtzi and the native Daphnia parvula. Daphnia lumholtzi had greater survivorship than D. parvula in most treatments and higher life-time fertility than D. parvula in all treatments. Both species achieved their fastest intrinsic rates of growth in treatments with high food, but their responses to suspended solids differed. The growth rate of D. lumholtzi in high food was slightly increased by higher turbidity, whereas that of D. parvula was depressed. 3. Results suggest that the ability of D. lumholtzi to tolerate suspended solids is an important factor contributing to its success in invading North American rivers.