The 96-h acute toxicity of Ba2+, o-Cresol and NaCl to Paratya australiensis was assessed in singular, binary and ternary combinations of these chemicals, in addition to three biochemical assays: glutathione S-transferase (GSTs), acetylcholinesterase (AChE) and sodium-potassium ATPase (Na+, K+-ATPase). The 96-h lethal concentrations that expressed 50 % mortality (LC50) in the singular toxicant exposures were Ba2+ = 23.4 mg/L, o-Cresol = 12.2 mg/L and NaCl 6936 mg/L. Toxic units were calculated and used to assess the binary and ternary interactions of the toxicants. A more than additive effect was observed in the binary combination of Ba2+ and o-Cresol and was generally the case for other combinations of toxicants. Inconclusive or non-significant differences in the biochemical assays were observed due to variability in activity across P. australiensis individuals. The Ba2+ concentrations found in co-produced waters in particular regions of the world are of notable concern, with some co-produced waters having concentrations of Ba2+ up to 13800 mg/L, an approximate 590-fold of the Ba2+ LC50 observed in P. australiensis in this study.