Two experiments were conducted to better understand the role of urea in pond fertilization. in experiment 1, urea dissolved in pond water and incubated in aquaria at approximately 30degrees Celsius disappeared at a rate of 303ug urea-N1 day under natural sunlight conditions, and 102 ug urea-N1 day under dark conditions. Total alkalinity decreased in dark aquaria and increased in sun-exposed aquaria, consistent with stoiciometric expectations. Urea hydrolysis was negligible in distilled water exposed to light. Experiment 2 was a toxicity test. Media lethal concentrations of urea and 24 and 96 h exposure were 19 700 mg 1 and 16 800 mg 1 for sex-reversed male Nile tilapia fingerlings, and 17 000 mg 1 and 17 000 mg 1 for silver barb fingerlings respectively. All test fish survived 96-h exposure to 14 000 mg urea 1 whereas no fish survived 48-h exposure to 22 000 mg urea 1.