Eutrophication-related problems in the Western Harbour of Alexandria were studied monthly from April 1999 to March 2000. Variation in salinity appeared to be the key to all changes in water quality and plankton abundance in the harbour. Both at the surface and near the bottom the salinity was lower (annual average: 35.1 and 38.3 PSU respectively) than in the open sea (39 PSU). Dissolved oxygen levels indicated poor aeration conditions along the water column (2.3-3.98 mg l^-1). Average pH values were approximately similar in the two layers (8.1 and 8 respectively) but exhibited different ranges of variations. Nutrient salts varied widely, often occurring in high concentrations, with ranges of 0.12-5.7 and 0.06-2.6 µM at the surface and the bottom respectively for phosphate, 0.21-20.46 and 0.25-18.12 µM for nitrate, 0.29-3.3 and 0.23-1.66 µM for nitrite, 0.56-57.46 and 2.32-43.73 µM for ammonia and 0.3-36.3 and 0.48-38.4 µM for silicate. As a result of nutrient enrichment, phytoplankton growth was very intensive, reflected by an abnormally high concentration of chlorophyll a (annual average: 33.82 µg l^-1). At the same time the death of large numbers of phytoplankton cells could be inferred from the relatively large amount of phaeopigment (annual average: 10.39 µg l^-1). The high levels of nutrient salts and phytoplankton biomass together serve as a good indicator of high eutrophication levels in the Western Harbour throughout the year. These conditions clearly affected the zooplankton stock, which varied between 5.8-93.6 × 10³ indiv. m^-3, although for most of the time values remained at a low level (annual average: 26 728 indiv. m^-3).