The development of visual acuity (minimum separable visual angle) in rainbow trout (Salmo gairdneri) was investigated from hatching to one year old trout by eliciting optomotor reactions in an optokinetic drum. I. The first optomotor responses were found 10 days after hatching with a visual angle of 30° of arc. Then a rapid increase in the visual resolving power occurs leading to a visual angle of about 1° at the end of the larval period. Up to the age of one year the visual angle of rainbow trout improves slightly to final 14' of arc. 2. Light deprivation during rearing causes a severe impairment ot visual acuity during the first 40 days after hatching (best visual angle only 2°05' of arc). 3. The results are discussed with respect to synaptogenetical and biochemical findings in the tectum opticum of rainbow trout under the same rearing conditions.