. A marine fish farm (production ~ 100 tonnes per year) located in the Orbetello lagoon area (Tuscany, Italy) was selected to study the waste water quality discharged. The aims of study were: a) quantify the nitrogen and phosphorus discharged from the fish farm at the beginning and the end of feeding time; b) verify nutrient trend during a daily cycle, and difference of release between nitrogen and phosphorus by reared biomass. From June 1999 to May 2000, water temperature, pH, salinity, dissolved oxygen and redox were hourly recorded in output wastewater station (E). Two water sampling strategies were adopted: monthly and daily. Monthly, in E, two field water samples at 8:00 a.m. (E8), at the beginning of feeding, and at 4:00 p.m. (E16), at the end of feeding, were collected. The dissolved and particulate nitrogen and phosphorus were analysed. With respect to total dissolved nitrogen (TDN), the total ammonia (NH3 + NH4+) concentration constituted more than 64%. With respect to total nitrogen and phosphorus, the particulated component represented, respectively, the 16% and 29–36%. A 24 h water sampling, at 4:00, 10:00, 12:00 a.m and at 4:00, 8:00, 12:00 p.m., was carried out. The results showed that TDN was excreted shortly after feeding, with a peak 4 h after the end of feeding. Most of the total dissolved phosphorus (TDP) was excreted after many hours, with a peak 8 h after the end of feeding.