We sequenced mRNA from the gill tissue of 84 individual fishes across 9 poeciliid species found in both hydrogen sulfide-rich springs and non-sulfidic environments (7 non-sulfidic populations, 7 sulfidic populations, 6 individuals per population 10 samples were sequenced twice to achieve similar depth of coverage across all samples). We used this RNAseq approach to identify differentially expressed genes between the sulfidic and non-sulfidic populations of each species, as well as to characterize patterns of molecular evolution to identify genes that have experienced positive selection in sulfidic lineages.