This paper reports the results of a research carried out on zooplankton in three adjacent environments in the Po River Delta: the terminal reach of the Po di Pila, the Sacca del Canarin, a small lagoon sited between Po di Pila and PO di Tolle, and the nearby stretch of sea. Samplings were effected monthly from April to December 1986. Biotic exchanges between river, lagoon and sea were evaluated. River zooplankton was dominated by Rotifers; maximum density (945 ind. l^-l) was reached in September. The salt water wedge intrusion in the river was noticed several times: neritic forms were found in some deep water samples, even at the station farthest away (about 7 km) from the river mouth. During the past few years, after the closing of the channel connecting the Sacca del Canarin to the Po di Pila, the lagoon has undergone great changes in its hydrological features, in the benthos structure, and in the composition and production of zooplankton populations. There has been a striking increase in the total zooplankton density; furthermore, this increase was due mainly to forms native to the lagoon. Rotifers (mainly Synchaeta spp., which reached densities over 1000 ind. l^-1 in May-June), Polychaete, Bivalve, Gastropod and Cirriped larvae, several species of Calanoids and juvenile stages of meiobenthic Copepods may be qualified as typical autochthonous components of the lagoon zooplankton. Cirriped larvae dispersal towards the sea was estimated. As a consequence of its recent evolution, the lagoon has been acquiring a more important role in the export to the sea of plankton biomass. Zooplankton composition in the coastal stretch, mainly at surface, was greatly influenced by the lagoon. Distribution of Polychaete and Bivalve larvae was compared to that of the corresponding macrobenthic taxa in the lagoon and in the sea.