During a study of the reproductive cycle of a subtidal population of Chaetozone setosa Malmgren at an 80 m station off the Northumberland coast (Christie, 1981), the discovery of morphological and reproductive differences in an intertidal population lead to an extended comparative study of three Northumberland populations. The two additional populations chosen for comparison are situated at Low Newton by the Sea and Holy Island, Northumberland. All three populations are located within a maximum distance of 65 km from each other which makes these populations very close on a geographical scale.