Microscopic plastic (MP) particles are a ubiquitous contaminant in aquatic environments, which may bind hydrophobic chemicals, such as polycyclic aromatic hydrocarbons (PAHs), altering their environmental fate and interactions with biota. Using rainbow trout gill (RTgill-W1) and intestinal (RTgutGC) epithelial cells we investigated the effects of polystyrene microbeads (PS-MBs; 220 nm) on the cyto- and genotoxicity of the environmental pollutants benzo[a]pyrene (BaP) and 3-nitrobenzanthrone (3-NBA) over 48 h (0, 0.1, 1 and 10 µM). The Alamar Blue bioassay, used to assess cytotoxicity, showed that both pollutants significantly decreased cell viability by 10-20% at 10 µM in both cell lines after 48 h whereas PS-MBs (5 or 50 µg mL-1) were non-toxic. Cytotoxicity in cells treated with PS-MBs together with BaP or 3-NBA were similar to those observed after exposure to BaP or 3-NBA alone. Using the formamidopyrimidine-DNA glycosylase (FPG)-modified comet assay 3-NBA, but not BaP, induced DNA damage in RTgutGC cells at 10 µM (10% tail DNA in the absence and 15% tail DNA in the presence of FPG versus 1% in controls), whereas PS-MBs alone showed no detrimental effects. Interestingly, comet formation was substantially increased (4-fold) when RTgutGC cells were exposed to PS-MBs (50 µg mL-1) and 10 µM 3-NBA compared to cells treated with 3-NBA alone. Further, using 32P-postlabelling we observed strong DNA adduct formation in 3-NBA-exposed RTgutGC cells (900 adducts/108 nucleotides). 3-NBA-derived DNA adduct formation was significantly decreased (20%) when RTgutGC cells were exposed to MB and 3-NBA compared to cells treated with 3-NBA alone. Our results show that PS-MBs impact on the genotoxicity of 3-NBA, causing a significant increase in DNA damage as measured by the comet assay in the intestinal cell line, providing proof of principle that MPs may alter the genotoxic potential of PAHs in fish cells.