Restriction-fragment analysis was used to measure mitochondrial DNA (mtDNA) variability in 79 individuals of two species of temperate sea urchins. For the purple urchin Strongylocentrotus purpuratus, individuals were collected 1,500 km apart in 1985 and again from the same localities in 1988 (about one urchin generation). Twenty mtDNA genotypes belonging to four clades were found among 38 individuals. All four clades were found in both localities and in both years. Genetic structure was further tested by calculating the degree of interdeme genetic variation (GST ) and comparing this value to the GST 's from randomly shuffled data. No geographic structure was found. For S. droebachiensis, only six mtDNA genotypes were found among 41 individuals collected from the Pacific and Atlantic oceans. More than 80% of the individuals belonged to two genotypes. The genotype that dominated collections in the Pacific also occurred in the Atlantic; however, a common Atlantic genotype was never found in the Pacific. These two genotypes were identical at 64 of 65 restriction sites, and were only 0.2% divergent from each other. GST analysis confirmed that there were significant genetic differences between Atlantic and Pacific populations. The small divergence between genotypes suggests recent, but not continuous, migration. These marine species show smaller genotypic differences than terrestrial species over similar spatial and temporal scales. Both recruitment of adults from planktonic larval pools and the spread of sibling larvae over large distances from parents probably act as buffers to genetic differences in species with planktonic life-history phases.