Research of phytoplankton communities development in the river Bosut was conducted in the period from February 2013 to July 2013 on two locations that are set upstream and downstream from the town of Vinkovci. Influences of temporal changes in physico-chemical parameters on quantitative phytoplankton composition were investigated. High concentrations of nitrates and total nitrogen in the water were determined during the low water levels, and the water level elevation was followed by high concentrations of orthophosphates and total amount of phosphorus. In the phytoplankton community structure green algae (from functional groups J, X1 and X2) and centric and pennate diatoms (functional groups D, P and TB) were the most abundant, while cyanobacteria were being occasionally represented. The phytoplankton communities composition is typical for slow-flowing reaches of lowland rivers. However, high growth rates of mixotrophic species such as Ochromonas sp. in May and June are the result of high levels of suspended materials in water that indicate the pollution of the body of water. Based on the given results, the moderate ecological status of the river Bosut was evaluated. Continuous monitoring of the biological indicators of the ecological status is essential for defining protective measures and the revitalization of the river Bosut.