1. The distribution of the isopod Asellus communis cannot be accounted for by its rheotactic reaction alone, but can be accounted for by the interacting thigmotactic and rheotactic reactions. 2. The movements given in rheotactic reactions of isopods are of secondary importance in their maintenance of position in streams. 3. The sign of the rheotactic reaction is of primary importance in this regard in that an isopod oriented positively can withstand approximately twice the current strength of one oriented in the opposite direction. 4. The clinging ability of positively oriented isopods varies somewhat with the degree, of positiveness of the rheotactic reaction which in turn is closely correlated with the metabolic conditions of the isopods. 5. The molting period and breeding season are especially important in the ecology of stream isopods in that they directly decrease the clinging ability and indirectly affect it by lowering the tendency to give positive rheotactic reactions. 6. Beyond possessing irritable protoplasm isopods are no more adapted to the stream environment by reason of their positive rheotaxis than the stream is adapted to the isopod by virtue of presenting the complex of environmental conditions best calculated to call forth and maintain positive rheotaxis.