Water and bottom sediment from different zones of the Głębokie eutrophic lake were subjected to 5-year studies (1986-1990). Activity of total alkaline phosphatase was determined four times a year in these environments. Studies upon water revealed that, among the zones analyzed, upper sub-littoral layer (0.5-1 m) was characterized by the highest phosphatase activity. For the bottom sediment, a higher level of the enzymatic parameter was recorded in the littoral. From the analysis of seasonal oscillations it follows that activity maximum for alkaline phosphatase, both in water and bottom sediment, occurred in spring (April) and summer (July). Basing on that parameter, no increase of eutrophization process in the Głębokie lake was observed during the 5-year period.