The occurence of tumors on the body of hydra has already been reported in researches but this is the first study in which the individuals of symbiotic green hydra and nonsymbiotic brown hydra were exposed to UVB-radiation and /or norflurazon in order to induce tumors. The individuals of green hydra (Hydra viridissima Pallas, 1766) and brown hydra (Hydra oligactis Pallas, 1766), either separately or both species together, were treated with norflurazon in concentrations of 2x10-7 mol/L or 2x10-6 mol/L. A group of hydra was treated with norflurazon and exposed to UVB-radiation with a wavelenght of 254 nm for 2 minutes. The second part of the experiment was performed by adding suspensions of free-living algae Chlorella vulgaris (CV) or endosimbiotic Mychonastes homosphaera (CZ10) and both algae together. The group of hydra with algae suspension was treated with norflurazon and exposed to UVB-radiation in the same way as in the first part of the experiment. The following morphological changes of hydra were observed: migration, response to mechanical stimuli, body shape, damage to tentacles, asexual (budding) and sexual reproduction, and mortality. The experimental dishes with algae suspension were monitored for the change of solution color and the presence of algae blobs. Migration, change of body shape from relaxed to predominantly contracted, and sexual reproduction were identified as the first evasion signs of the deleterious effects of UVB-radiation and norflurazon. Histological changes were detected by light microscope, while ectoderm, gastroderm, interstitial, zymogen and cnidocyte cells were counted. The thickness of mesoglea was measured. Mortality was present in the individuals of green hydra that were exposed to UVB-radiation. Severe damage to the structure of hydra tissue was noted in hydra treated with high concentrations of norflurazon and in all hydra exposed to UVB-radiation. Radiation caused tissue growths on both tentacles and body of hydra as well as specific fork-shaped tentacles in hydra.