The genus Thunnus is within the family Scombridae (mackerels and tunas). Morphological and molecular systematic analyses of the tuna species have resulted in conflicting phylogenetic trees. The most recent molecular phylogenetic analysis is based on evidence obtained from the mitochondrial cytochrome b gene. Since mitochondrial DNA is maternally inherited, it represents a historical record of only half the population. Our research is focused on characterizing and utilizing nuclear repetitive DNA sequences as phylogenetic markers to help resolve this conflict. We have recently isolated one hypervariable microsatellite locus and a satellite DNA sequence from yellowfin tuna genomic DNA. Our next step will be to characterize the satellite sequence which is conserved in yellowfin, bluefin and albacore tuna and use it to initiate a systematic study of the phylogenetic relations of the tuna species.