Genetic differentiation in the New Zealand sea urchinEvechinus chloroticus(Valenciennes) was studied by examining individuals collected from six widely separated (250–2200 km apart) sites from around New Zealand for genetic variation at five polymorphic enzyme loci. Despite the large geographic separations, values of Nei's unbiased genetic distance,D(0–0.019) and standardised genetic variation among populations,FST(0.01–0.02) were small. This indicates high levels of gene flow among populations. However, one population sampled in a fiord in south‐western New Zealand was genetically differentiated from the others, although the scale of differentiation was not large (D =0.011–0.019). These findings indicate: (1) that the long‐lived planktotrophic larva ofE. chloroticusprovides this species with considerable dispersal ability throughout its range; and (2) that this is little affected by oceanographic features peculiar to its range, such as the Subtropical Convergence or the southward flowing East Cape Current. The slight differentiation of the fiord population probably results from restricted larval dispersal into and out of the fiord. In terms of management of theE. chloroticusfishery, the results provide no evidence for discrete stocks, with the possible exception of Fiordland. It is prudent to manage the fiord populations conservatively until they are better studied.