Allelochemicals from eucalyptus released into the environment, mainly by leaves, can have a toxic effect on local biota, including in aquatic environments. Therefore, the present study evaluates the toxic activity of the water containing leaves of Eucalyptus urophylla S.T. Blake (Myrtaceae) in decomposition using Allium cepa L. (Amaryllidaceae) as a test organism. The toxicity and the cytotoxicity evaluation were performed using onion bulbs (A. cepa). The toxicity was assessed by inhibiting root growth. The cytotoxicity was evaluated by using a comparison of the mitotic index (MI) and the negative control. The extraction of eucalyptus metabolites from water of the decomposition tests and creek water was performed by solid-phase microextraction (SPME). The chemical characterization was done by gas chromatography coupled to mass spectrometry (GC-MS). There was inhibition of the root growth of A. cepa, indicating toxicity of the compounds released in the water during the decomposition. The cytotoxicity tests did not indicate a toxic effect. However, there were identified some mutations, cell death, and morphological changes in the roots. 26 compounds were identified on samples of water acquired from decomposition tests.Fenchone, 2-ethyl-1-hexanol, cis-dihydrocarvone, and trans-dihydrocarvone were identified in all samples. The results highlight the importance of studies and monitoring of aquatic environments near eucalyptus.