The paper presents the quantity and quality (fractions) of dissolved organic carbon (DOC) in shallow ground waters of arable fields and littoral lake water and their seasonal changes. The study was conducted in the catchment of mesotrophic Piaseczno Lake in years 1992–1998. The samples of ground waters were taken from the observation well installed on the arable area about 15–20 m from the shoreline, on the border between the fields and the beach. Ground and lake water for analyses were taken in spring (IV, V), summer (VI, VII) and autumn (IX, X). In the water samples total dissolved organic carbon (DOC) and its fractions were determined. The fractions of DOC were the following: humus acids extracted in 0.5 mol NaOH dm-3, hemicelluloses dissolved in 0.5 mol H2SO4 dm-3 and fraction not yielding to extraction, i.e. residue. A program STATISTICA was used for statistical analyses. It was stated that the concentration of DOC and C of the humus acids in ground waters was significantly higher than in the lake waters. However, no differences were found between waters when percentage contribution of C in particular fractions to total DOC concentration was considered. In both kind of waters the contribution of humus acids amounted to about 60%, hemicelluloses to 7–8% and residue to 30% of DOC. Seasonal changes in the concentration of DOC and C of the fractions in waters were statistically not significant. However, a tendency for its higher values in autumn in comparison with summer and spring were observed. The results suggest that the ground waters from agricultural part of the catchment have influence on the state of dissolved organic carbon in the lake waters.