The macrophyte biological index for rivers (IBMR) is the only method which describes eutrophication as well as organic pollution in a single index value. Presence of Hippuris vulgaris in water or lack thereof may affect the IBMR value. Potential effects of water pollution with Decis 2.5 EC containing deltamethrin in a water/sediment/plant system were the objectives of this research because this insecticide often occurs in soil and water as a contamination. Therefore, the aim of the present study was to answer the question of how Decis 2.5 EC as stress factor affects the growth of H. vulgaris, and determine the persistence of deltamethrin in the water/sediment/plant system. This study was conducted under different trophic levels during 18 wk in each of 3 consecutive years. The half-life of deltamethrin in water was estimated in the test system. It was found that H. vulgaris demonstrated its ability to survive in the water/sediment environment contaminated with Decis 2.5 EC despite the inhibition of the plant's development. In the presence of insecticidal formulation, the plant's growth was much slower compared to control. The tolerance index ranged from 0.19 to 0.37 with the lower values obtained for the higher concentration of phosphorus, nitrogen, potassium. Long-term studies of H. vulgaris grown under different trophic conditions indicate that despite the plant's ability to survive the contamination with Decis 2.5 EC, the timing of the macrophytes assessment is crucial to assess the occurrence and development of this plant and the ecological status of water.