A pilot investigation conducted at the City of Roseville, California evaluated the use of water hyacinth aquatic plants as a post-secondary wastewater treatment process. These systems substantially reduced ammonia concentrations during warm seasons by more than 70% in nonaerated ponds and 99% in a pond with supplemental aeration. Total nitrogen reductions in warm seasons were approximately 55 and 70% for the nonaerated and aerated ponds, respectively. Effluent BOD5 and SS levels were typically below 10 mg/l. Winter dieback of hyacinths led to excessive organic loading in late winter and spring and caused production of sulfides and concomitant odors. Harvesting and aeration were effective in minimizing or preventing such conditions.