The aim of this study was to determine the effects of three linear cyanobacterial oligopeptides: microginin-FR1 (MG-FR1), anabaenopeptin-B (ANA-B), aeruginosin 98A (AER-A), aeruginosin 98B (AER-B) and one cyclic oligopeptide anabaenopeptin-B (ANA-B) on RTgill-W1 fluorescent biomarkers of cell line derived from Rainbow trout gill cells. The following cellular parameters were determined after 48-h exposure to different concentrations (205 nM, 512 nM and 1004 nM) of the oligopeptides: Hoechst and Propidium Iodide (PI) fluorescence, intracellular glutathione level and cytoskeletal F-actin structure. The study showed that AER-B and MG-FR1 were the most potent inhibitors of Hoechst fluorescence, AER-B additionally increased PI fluorescence level. MG- FR1 and ANA-B substantially reduced glutathione and F-actin fluorescence level in RTgill-W1 cells. The obtained results suggest that both linear and cyanobacterial oligopeptides should be considered as factors inducing detrimental alterations in RTgill-W1 cells, suggesting that in natural conditions exposure of fish to cyanobacterial metabolites may result in cytotoxic effects in fish gills.