The experiments of Lyon ('o5a., '05b, 'o9) seem to show that rheotropism in fishes is a reaction which is brought about by a sense of relative motion. In a uniform current the fish heads upstream, not because it is under an actual current stimulus, but because it is attempting to maintain a constant visual image while the current is acting to carry the fish downstream. Garrey ('o5) approached the problem from a different angle, but his results seem to dovetail with those of Lyon. Both of these experimenters were able to alter the orientation of the fishes by moving objects outside the aquarium. The fishes followed the moving objects in order to keep the same optical image. With a view to carrying forward these experiments and in an attempt to attach to them some ecological meaning, the following experiment was carried on. STATEMENT OF THE PROBLEM