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Polyaromatic hydrocarbons (PAH) are common pollutants of water ecosystems originating from incineration processes and contamination with mineral oil. Water solubility of PAHs is generally low; for toxicity tests with aquatic organisms, they are therefore usually dissolved in organic solvents. Here we examined the effects of a typical model PAH, phenanthrene, and a solvent, acetone, on amphipods as relevant aquatic invertebrate models. Two of these species, Eulimnogammarus verrucosus and Eulimnogammarus cyaneus , are common endemics of the oligotrophic and pristine Lake Baikal, while one, Gammarus lacustris , is widespread throughout the Holarctic and inhabits smaller and more eutrophic water bodies in the Baikal area. Neither solvent nor phenanthrene caused mortality at the applied concentrations, but both substances affected gene expression in all species. Differential gene expression was more profound in the species from Lake Baikal than in the Holarctic species. Moreover, in one of the Baikal species, E. cyaneus , we found that many known components of the cellular xenobiotic detoxification system reacted to the treatments. Finally, we detected a negative relationship between changes in transcript abundances in response to the solvent and phenanthrene. This mixture effect, weaker than the impact by a single mixture component, needs further exploration. Unlabelled Image • Available studies on polyaromatic hydrocarbons in Baikal amphipods are scarce. • 1 mg/mL phenanthrene and 0.1% acetone did not cause mortality in the studied species. • The solvent (acetone) had a stronger effect on gene expression than phenanthrene. • Transcriptional responses to the solvent and phenanthrene are negatively correlated. [ABSTRACT FROM AUTHOR]