Boron is an essential nutrient for plants and an essential element for many organisms, but can be toxic to aquatic and terrestrial organisms above certain concentrations. The aim of this research was to determine boron concentrations in water, sediment and biotic samples (Gammaridae spp.-Crustacea, Helix sp.-Gastropoda, Donax sp.-Bivalvia, Helobdella sp.-Hirudinae, Ephemeroptera nymph, Chrinomidae larvae, Tipulidae larvae-Insecta, Rana sp.-Amphibia, Natrix sp.-Serpentes, fish sample Leiscus cephalus (Linnaeus, 1758) and leaves of Salix sp.-Salicacea from Seydi Stream (Kirka-Eskişehir). Our results have shown that boron concentrations of the Seydi Stream water is higher than the Turkish Environmental Guidelines standard (>1 mg L(-1)) and in Europe (mean values typically below 0.6 mg L(-1)).