The mechanisms and pathways by which nitrogen in its various forms is removed from waste stabilization ponds (WSP) have been a subject of much debate. In order to improve current understanding of the dynamics of inorganic and organic nitrogen removal in WSP systems, a study using stable nitrogen isotopes (15N) was undertaken on an experimental pilot-scale WSP system at Esholt Wastewater Treatment Works (Bradford, UK) in summer 2005 and 2006. A primary maturation pond was spiked with 15N-labelled ammonia (15NH4Cl) and 15N-labelled algae (Chlorella vulgaris) to track nitrogen transformations and removal associated with ammonia volatilization, nitrification, and algal uptake and its subsequent sedimentation and retention/hydrolysis in the sludge layer. Stable isotope analysis of 15N, in conjunction with performance indicators, revealed that total nitrogen is poorly removed (? 8%) but ammonium nitrogen removal is very good in summer (effluent concentration <2 mg NH4 + -N/L). The nitrogen cycle in maturation ponds is dominated by biological uptake as ammonium nitrogen is rapidly transformed into algal biomass as suspended organic nitrogen which then either leaves in the pond effluent or is sedimented as dead cells. Ammonia removal by volatilization makes little or no contribution to nitrogen removal and biological nitrate uptake prevents classical nitrification. Thus algal uptake of ammonium and subsequent sedimentation and retention in the sludge layer, after parti al ammonification of the algal organic nitrogen, appears to be the dominant mechanism for permanent nitrogen removal in maturation ponds during warm summer months in England.