A study on the factors influencing nitrogen removal in waste water stabilization ponds was undertaken in an eight-pond series in Werribee, Australia. Nitrogen species including Kjeldahl nitrogen, total ammonia nitrogen, nitrite and nitrate were monitored monthly from March 1993 to January 1994. At the same time, pH, temperature, chlorophylla content and dissolved oxygen were also recorded. Highest nitrogen removal occurred during the period with highest levels of chlorophylla content and dissolved oxygen, but the rate of nitrogen removal was not related to temperature and pH. Enhanced photosynthetic activities resulting from an increased phytoplankton abundance due to prolonged detention time caused an increase in dissolved oxygen, and created an optimum condition for nitrification to occur. In this process, ammonia was oxidized to nitrite and nitrate which were subsequently reduced to elemental nitrogen. Apart from nitrification-denitrification which was the major nitrogen removal pathway in the study system, algal uptake of ammonium, nitrate and nitrite as nutrient sources also contributed to the nitrogen removal. The role of phytoplankton and zooplankton in the treatment process in waste stabilization ponds was discussed.