Wimpenny's measurements of Rhizosolenia styliformis (this Journal, Vol. xxi, pp. 29-60,1936) are used to discuss the value of population curves of this species as indicators of water movements and mixture in the North Sea. His alleged correlation between average size and temperature is shown in this case to be untenable. On the coastal banks during 1933 there was a steady increase of size accompanying normal regeneration (bimodal curves), but this was complete by November, and in 1934, with fuller seasonal sampling, the size steadily diminished (all curves unimodal). The population phases on the Dogger and coastal banks were markedly divergent in 1933, but almost identical and intermixed in 1934, indicating a northern (“Atlantic”) influx in 1933 and its absence in 1934. Off the southern edge of the Dogger in the autumn of 1932 a bimodal and unimodal population were variously mixed, the former showing relations to the coastal population of the following spring, the latter probably marking the intrusive northern element which persisted into 1933.