Water quality of the liwiec River (the longest tributary of the Bug River, the South Podlasie Lowland, and the Central Mazovia Lowland) was evaluated in 1998-2000 and 2002 using selected physical and chemical parameters and macroinvertebrate analysis. Classification of water quality was done on the basis of physical and chemical parameters. Taxonomic composition of invertebrate macrofauna was used for calculation of the following biological indices: Belgian Biological Index (BBI), British BMWP/OQR Index, and modified for Polish rivers, Margaleff's index of biological diversity. The values of physical and chemical parameters showed that water of the Liwiec River belongs to the 2^nd and 3^rd class of quality (in the five degree scale). The values of BBI and BMWP/OQR indices revealed that the water was moderately polluted. According to the biodiversity index, the Liwiec River was classified as the 1^st quality class. Correlation between chemical parameters and BBI and BMWP/OQR values shows that these indices may be used for evaluation of water quality in Polish lowland rivers (like Liwiec). However, in the case of the biodiversity index, the ranges for various water quality classes should be modified.