During investigations into the endocrinology of Leander ( =Palaemon) serratus (Pennant) several differences in physiology (which will be reported in this Journal at a later date) have been found between the prawns from Plymouth and those from Roscoff on the north coast of France. These experiments involved the repeated handling and the close examination of about a thousand prawns from Roscoff and many more from Plymouth. Increasing familiarity with these animals led to the realization that the two samples could readily be distinguished by eye. It was not always possible to determine the provenance of a single individual in this way, but any group of five or six could be told at a glance. A detailed examination of the external morphology showed that the characteristic differences by which the provenance of these animals could be determined lay not in any anatomical feature but in the disposition and arrangement of the chromatophores which form the pattern on the dorsal side of the cephalothoracic shield and the anterior part of the abdomen. Great individual variations were to be seen, but differing trends of development of the patterns were found in the two populations. There was some degree of overlap between the populations, but only about 15% of the prawns lay in this region of overlap and thus failed to bear the marks of their locality; the remaining 85% could be assigned to either Roscoff or Plymouth with little difficulty. Recently I have been able to examine about forty prawns from Concarneau, on the Atlantic coast of France, and these again are mostly different from either of the other two samples.