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Abstract Freshwater pollution is a major global concern. Common methods for determining the effects of contaminants on freshwater organisms involve short-term laboratory experiments with otherwise healthy organisms. However, in natural systems, organisms are commonly exposed to parasites, which could alter their ability to survive exposure to aquatic contamination. We used a freshwater crustacean (Daphnia dentifera) to quantify the effects of parasite exposure on mortality from two common freshwater contaminants (elevated salinity [NaCl] and carbaryl). In our salinity trial, both parasite exposure and elevated salinity reduced survival in an additive manner. In our carbaryl trial, exposure to carbaryl reduced survival and we found a less-than-additive (i.e. antagonistic) interaction between carbaryl and the parasite; the parasite only reduced survival in the control (no carbaryl) treatments. Our results demonstrate that parasites and contaminants can jointly affect mortality in aquatic organisms in an additive or less-than-additive manner.