the study of ecotoxicology inevitably must address the fact that toxicants always occur in mixture. unfortunately, our present state of knowledge in terms of predicting the effects of contaminant mixtures and understanding the mechanisms by which a mixture can produce non-additive (e.g., antagonistic or synergistic) toxicity is insufficient to advise regulatory authorities on appropriate water quality objectives for the protection of aquatic life. metals and polycyclic aromatic hydrocarbons (pahs) are two ubiquitous contaminants that are often associated with similar effluent sources, such as bitumen and municipal waste. the adverse toxicological effects that metals and pahs produce have given these contaminants the status of “priority pollutants” in many countries, including the usa and canada. thus, understanding their toxicological effects when in mixture should also be a priority. however, to date, there have been only 11 studies that have investigated the potential for metals and pahs to produce non-additive toxicity. of these 11 studies, reports of more-than-additive lethality have been equally common as strictly-additive lethality, raising concern over the largely ignored ecological risk these contaminants types produce when in mixture. the research outlined in this dissertation expands our understanding by providing the first comprehensive review of mechanistic aspects of metal-pah mixture toxicity that likely amount to more-than-additive co-toxicity. this dissertation outlines experimental work investigating the additivity of binary mixtures of cu, cd, ni, and v, with either phenanthrene (phe) or phenanthrenequinone (phq), two common pahs. for cases where more-than-additive mortality was found, additional experimentation was carried out to explore interactive toxic mechanisms in attempt to explain why certain mixtures of metals and pahs produce more-than-additive lethality. finally, the effects of cu, phe, and cu-phe mixtures were studied in terms of their effects on behaviour, a sublethal endpoint that mediates ecological effects which can also be used to predict toxic mechanisms. all experimental work outlined in this dissertation involved the aquatic crustacean amphipod, hyalella azteca, which was selected due to its tractability in a laboratory setting, its ecological importance as a food source for fish, amphibians, and waterfowl, and its widespread distribution throughout north and central america.