The presence of several anthropogenic chemicals has been documented in the atmosphere of the Canadian prairies. The deposition of these chemicals as a mixture is of importance since little is known of the combined effects of these chemicals on aquatic organisms. This study was designed to evaluate the acute and chronic toxicity of a complex mixture of nine atmospherically transported pesticides to Ceriodaphnia dubia. The nine selected pesticides (bromoxynil, dicamba, 2,4-D, MCPA, triallate, trifluralin, pentachlorophenol, lindane, and 4,4'-DDT) were detected in appreciable quantities in dry atmospheric deposits. The concentration of each pesticide in the mixture was based on maximum measured daily dry deposition rates for central Canada, except for pentachlorophenol, which was estimated based on atmospheric concentrations. The 48-h LC50 estimate for C. dubia exposed to the pesticide mixture was 174.60 microg L(-1) (340 times the measured total dry deposition concentration). The estimated NOEC and LOEC for both survival and reproduction, as determined in the 7-d chronic toxicity test, were 51.3 (100 times) and 154 microg/L(-1) (300 times), respectively. A basic risk assessment, using the toxic unit approach, suggested that the toxicity of the pesticide mixture was mainly due to 4,4'-DDT. Overall, this atmospherically transported complex mixture of pesticides appears to pose a negligible toxicological risk to non-target aquatic invertebrates such as zooplankton.