Retinal structure and concentration of retinoids involved in phototransduction changed during early development of rainbow trout Oncorhynchus mykiss, correlating with improvements in visual function. A test chamber was used to evaluate the presence of optokinetic or optomotor responses and to assess the functionality of the integrated cellular, physiological and biochemical components of the visual system. The results indicated that in rainbow trout optomotor responses start at 10 days post-hatch, and demonstrated for the first time that increases in acuity, sensitivity to low light as well as in motion detection abilities occur from this stage until exogenous feeding starts. The structure of retinal cells such as cone ellipsoids increased in length as photopic visual acuity improved, and rod densities increased concurrently with improvements in scotopic thresholds (2.2 log10 units). An increase in the concentrations of the chromophore all-trans-retinal correlated with improvements of all behavioural measures of visual function during the same developmental phase. ?? 2004 The Fisheries Society of the British Isles.