Molecular analyses of the mitochondrial genome using restriction fragment length polymorphism (RFLP) were utilized to determine the existence of discrete populations of the tidepool sculpin Oligocottus maculosus from the Barkley Sound region of southβwestern Vancouver Island, British Columbia. Fiftyβfive haplotypes were found with three haplotypes making up 38, 16 and 8% of the total sample, respectively. Within individual tidepools, haplotype diversity ranged from 0Β·30 to 0Β·92, but haplotypes were typically closely related to one another (nucleotide diversity ranged from 0Β·0004 to 0Β·005). The majority of haplotype variation (96β97%) was located within tidepools, but a significant amount of variation (c. 3β4%) was detected among tidepools within single islands. By contrast, there was no significant differentiation among islands or between samples from the Barkley Sound area and a single sample from northβeastern Vancouver Island. The results suggest a low level of population structure within the sample area and that the distribution of haplotypes and haplotype divergences represents a demographic expansion from a single ancestral lineage beginning between 300β000 and 1β000β000 years ago. The low level of population subdivision resolved suggests that despite their ecological specialization in tidepool habitats, tidepool sculpins are influenced by demographic and genetic processes that operate on spatial scales much larger than single tidepools or islands.