Muscle samples were collected from southern bluefin tunaThunnus maccoyiioff New Zealand, from northern bluefin tunaT. thynnusoff Japan, and from specimens recorded as northern bluefin tuna off New Zealand. Electrophoretic analyses of muscle tissues fromT. thynnusandT. maccoyiifound 2 out of 33 enzyme loci with species‐specific gel phenotypes. These two markers were used to identify specimens recorded as “northern” bluefin tuna in the New Zealand fishery. Only two out of 17 “northern” specimens wereT. thynnus.