Porbeagles (Lamna nasus) are a highly commercially important shark species that is threatened with extinction due to overfishing. Mitochondrial DNA (mtDNA) displacement loop (D-loop) sequence data from 18 Northwest (NW) Atlantic and 30 Southwest (SW) Pacific porbeagles reveal that these regional populations have been genetically separated between 1.39 and 1.25 million years ago (MYA), a time frame which correlates with the end of the earth’s last cooling period. There is far greater genetic differentiation (FST = 0.835) between the NW and SW populations than among sharks within each population supporting a very high level of divergence. A lack of gene flow probably stemming from their limited distribution to cold water temperatures (-1oC to 15oC) has led to their genetic divergence. The NW Atlantic population exhibited fewer haplotypes than the SW Pacific population (2 vs 4). The mean nucleotide diversity value of the NW Atlantic population was also 50% lower (0.00143 vs. 0.00228). Male and female NW Atlantic individuals reflected virtually identical mean population diversity values (0.00393 vs 0.00399); however, females were prevalent near shorelines while the males were more often found in open waters. Of the three age groups within the NW Atlantic population, the immature individuals exhibited the greatest mean nucleotide diversity (0.00452), followed by the sub-adult group (0.00293) and the mature group (0.00288), suggesting that dispersion starts earlier in their life cycle and reduces as they get older. The porbeagle population biology, as revealed by D-loop sequence information, may have significant implications for the conservation efforts of this species. As differences in age-based and sex-based dispersion exist, it is important to understand the relative contributions of gene flow by adults of both sexes in order to implement more effective conservation strategies.