Enclosures were placed in a pond in order to determine the effects of p-chloroaniline on the plankton communities and on the limnological conditions. In this way it was possible to approach almost natural conditions (creating mesocosms). The chemical was introduced into the enclosed aquatic biotopes with a novel slow release mechanism. The enclosures were allowed to equilibrate for 20 days before the chemical was applied. The results showed that p-chloroaniline caused a reduction of O2-concentration, pH- and conductivity values compared to controls. Phytoplankton was not significantly affected, b