1. The paper describes the distribution of A. aquaticus (L.) and A. meridianus Rac. in Windermere. 2. As far as Asellus is concerned the lake shores can be divided into northern, middle and southern zones. The middle zone shorelines are dominated by aquaticus, the southern and northern by meridianus. 3. No physical or chemical factor could be found that explained this distribution, but it was noticed that aquaticus tended to be associated with glacial drift shorelines and meridianus with Bannisdale slate shorelines. 4. Since there is more residential and human activity on the drift than on the Bannisdale shores, the association of aquaticus with drift shores may be due to its introduction by man. 5. A. aquaticus is regarded as a competitor to meridianus spreading along the shoreline from centres of human activity. 6. Below 2 m meridianus is the dominant form although the adjoining shore may be completely dominated by aquaticus. 7. The suggestion is made that meridianus is more sub-littoral than littoral, and unable to compete with aquaticus in the littoral region.