Between 1996 and 2016 the activity of alkaline phosphatase in the littoral water of Lake Piaseczno was researched in five-year measurement cycles. Samples of water from the area corresponding to the agricultural sector of the lake catchment area were used as the research material. Simultaneously, the content of nitrogen and phosphorus compounds in the soils of the agricultural sector of Lake Piaseczno was investigated. Between 1996 and 2006 the activity of alkaline phosphatase in the water under study was similar and exhibited a slight increasing tendency. In the consecutive measurement cycles (2011 and 2016) the activity of the enzyme in the water increased significantly. The research also proved that every year the con-tent of mineral forms of nitrogen and assimilable phosphorus in the soil surrounding the lake tended to increase. The enzy-matic parameter under study was used as an indicator of lake eutrophication. The research revealed that during the period under analysis the trophicity of Lake Piaseczno increased. .