We quantify genetic variation in components of OXPHOS and associated H2S detoxification enzymes in natural populations of Poecilia mexicana from sulfidic and non-sulfidic habitats. Our targets include approximately mitochondrial and nuclear genes, including all subunits of OXPHOS components, ATP synthase, and enzymes associated with the SQR pathway, as well as relevant assembly factors. Genetic variation at these loci was assayed in N=30 individuals per population (for a total of N=200) through targeted capture sequencing. For comparisons, we will also sequenced genes that are nuclear-encoded, mitochondrial proteins (matched for length and identity), but do not play a role in the targeted pathways.