The discharge of urinary and fecal phosphorus (P) from fish farms is a major environmental concern. In this study, urine P excretion in a potential aquaculture species, haddock was quantified and compared with values for Atlantic salmon. Fish were fed a commercial haddock diet containing 1.4% phosphorus, at a rate of 3% body weight for 3 weeks. Urine was collected four times a day (0, 3, 6 and 17 h after feeding). Urine volume, urine pH, urine phosphate concentration, and the relationship between urine volume and amount of food in the gut during urine collection were measured. Haddock and salmon excreted the highest quantity of urine at 3 and 6 h after ingestion of a meal. The total volume of urine measured at these periods was 83.0Â±5.18 and 243.8Â±10.51 Âµl kg-1 body weight (BW) urine, respectively. Urine phosphate concentrations of haddock and salmon were 4.6Â±0.08 and 1.0Â±0.08 mmol l-1 and urine pH was 6.2Â±0.25 and 7.5Â±0.01, respectively. The maximum urine volume was observed at 3-h post-feeding in haddock and 6 h in Atlantic salmon. The relationship between haddock urine volume and the gut content was best fitted by the quadratic equation: Haddock urine volume (Âµl kg-1 BW)=155.6–4.68 (% gut content)+ 0.049 (% gut content)2. This experiment was repeated and similar results were obtained.