An 84-day experiment assessed the combined effects of two fresh water quality levels (H: 1.71 +/- 0.15 mg O(2)l(-1) and 0.28 +/- 0.01 mg l(-1) total ammonia nitrogen (T-AN), L: 5.15 +/- 0.07 mg O2l(-1) and 0.54 +/- 0.01 mg T- AN l(-1)) and 3 stabilized stocking densities: 24.8 +/- 0.2, 74.2 +/- 0.5 and 120.0 +/- 0.9 kg m(-3)) on rainbow trout. Fish were fed using demand feeders with rewards proportional to stocking density. Mass increase was significantly affected by water quality and stocking density, being highest in H water and the lowest at 120 kg m(-3). There was no significant difference in final weight between 25 and 74 kg m(-3), but at 120 kg m(-3) it was 27% and 19% lower in H and L water respectively than at 25 kg m(-3). Feed intake (FI) from day 0-85 was significantly affected by water quality, 1.5% in H compared to 1.1-1.0 in L, but there were no significant differences in apparent feed conversion (AFC). FI was not significantly affected by stocking density but AFC was impaired, it increased with stocking density. Marked changes in fish morphology and composition were related to water quality: Condition K factor and fillet fat content were significantly higher in H than in L groups. Dorsal and pectoral fin condition was affected by stocking density and water quality: fins were significantly longer and less eroded in L groups and at low stocking density. Physiological measures were within the usual ranges, but differed between treatments. Changes in plasma osmolarity, hydromineral balance (Na+) and acid base balance (HCO3-) showed that fish were more affected by water quality than by stocking density. There was no sign of acute stress in acclimated fish as cortisol and glycemia were similar under all experimental conditions. This study highlights the importance of water quality and feeding conditions when considering the effects of stocking density on fish welfare.