One of the extreme situations that put people’s lives and possesions in danger are fires. During a fire and the subsequent disposal there is a release of dangerous substances, which negatively affect different components of the environment. A result of the extuingishing is that a secondary contamination with said substances occurs in the soil, air and water sources. A fire can result from different causes and has a devastating effect on the environment. Because of this reason it is imporant that the firefighters act quickly and effectively when they intervene, so that the negative impact resulting from the fire is reduced. During the firefighters intervention different materials are used, according to the type of fire. The choice of materials can already prevent or mitigate the negative effects on the environment and because of this reason it is important to pay attention to the effects of the materials on the components of the environment. This thesis deals with the determination of the toxicity of water used for fire extinguishing by using biotests of acute toxicity on Daphnia magna, inhibition of the growth of Lemna minor in an aquatic environment and the inhibition of the root growth of the higher culture plant Sinapis alba. The analyzed extinguishing water was from experimental simulated fires of natural substances (cardboard (KA), wood (D), straw (S), wood + straw (D + S) and wood + straw + cardboard (D + S + KA)) extinguished with water from a fire hydrant network. Most sensitive was the effect of the extinguishing water on Daphnia magna. All samples were positive with high immobilization of subjects. A significant toxic effect of the extinguishing water was also visible on Lemna minor, but without clear significant differences. Extinguishing water did not have a toxic effect in the terrestrial test with Sinapis alba. The results were negative. From said results follows, that the extinguishing water as a contaminator of soil is a lower environmental risk in comparison with an aquatic environment. By testing the phytoxicity of the extinguishing water when applied to soil, the inhibitory effect with an impact on the extinguished natural substance was confirmed.