The water quality behaviour of 58 stormwater retention basins in Winnipeg, Canada, was intensively studied during a 5-month summer period (May to September). Dissolved oxygen (DO), temperature, turbidity, transparency and depth were measured onsite. Samples analyzed in the laboratory included: total suspended solids (TSS), pH, chlorophyll a, fecal coliforms (FC), total kjeldahl nitrogen (TKN), ammonia, nitrate, total phosphorus (TP) and orthophosphate. Results showed that increases in TSS and turbidity resulted from increased chlorophyll a concentrations associated with algal growth. TKN increases were partly the result of nitrogen incorporation into proteinaceous material in the algal cells. Ammonia concentrations showed a maximum attributed to chemo-heterotrophic degradation of dead biomass. Orthophosphate made up 30% to 50% of the total phosphorus present and the two followed the same general trends. Temperature increased and then decreased over the summer months with no apparent thermal stratification. After starting the summer with no dis-solved oxygen gradient between top and bottom, depletion of the dissolved oxygen at the bottom of the basins during the latter half of the summer was attributed to chemoheterotrophic degradation of dead biomass. Finally, sub-stantial counts of fecal coliform bacteria occurred in the basins in response to Canada geese migration.WAKELIN, S. ; ELEFSINIOTIS, P. ; WAREHAM, D. (2003): Assessment of Stormwater Retention Basin Water Quality in Winnipeg, Canada. In: Water Quality Research Journal of Canada: Volume 38 , 433-450.