Composition and spatio-temporal distribution of zooplankton have been studied from 1977-79 in two lagoons of the Po River delta, Sacca di Scardovari and Sacca del Canarin. In Sacca di Scardovari, zooplankton are very abundant and show a strong component of autochthonous meroplankton represented by larvae of polychaetes, molluscs and cirripedes; hydrodynamic factors, such as sea and river currents are a bad influence on biocoenosis variability. In contrast, Sacca del Canarin zooplankton has low densities, and it is principally represented by neritic forms; the autochthonous zooplankton component includes almost exclusively cirriped larvae, while polychaete and mollusc larvae come mainly into the lagoon from the sea; the biocoenosis variability is largely influenced by hydrodynamic factors.