Various populations of F. heteroclitus, a common killifish, suffer from devastating heart defects due to pollutants such as polychlorinated biphenyls (PCBs) and other Poly Aromatic Hydrocarbons (PAHs). F. heteroclitus from highly polluted populations in Massachusetts have evolved resistance to these pollutants, avoiding heart defects. Resistance has been hypothesized to occur in one of the two Aryl Hydrocarbon Receptors (AHR 1/2), a protein that brings the pollutants into the fish’s cells acting as a transcription factor for the Cytochrome P450 family of enzymes. Exon 10 of AHR1 contains several non-synonymous mutations that have been thought to contribute to resistance. The F. heteroclitus born in Darby Creek coursing through the John Heinz National Wildlife Refuge, Philadelphia, PA develop downstream from two EPA Superfund sites that leach PCBs and other contaminants into the Darby Creek. Through single nucleotide polymorphism (SNP) and direct sequence analyses of the mitochondrial DNA control region we determine whether the Heinz population, a nearby control population, and a coastal population consist of exclusively the northern or southern subspecies or a mixture of both as pollution responses may differ between subspecies. We are examining exon 10 of the AHR1 gene in all three populations to determine if nonsynonymous mutations have occurred, potentially conferring pollution resistance. Preliminary analysis reveals AHR1 exon 10 sequence differences among populations. Through identifying and studying F. heteroclitus, we investigate how pollution may be driving evolution.