Efforts to determine the utilization of Gulf of Mexico (GOM) chemosynthetic production by benthic predators have relied on stable isotope differences between photosynthetic and chemosynthetic production. Whereas the photosynthetic δ13C value in GOM surface waters is relatively uniform, chemosynthetic production may differ in different areas depending on prevalence of thiotrophy versus methanotrophy and inorganic carbon source. In this paper we compare the δ13C and δ15N signatures of the symbiont-containing mussel, Bathymodiolus childressi Gustafson, 1998, from four different chemosynthetic sites to test the hypothesis that methanotrophic production results in significant differences among them. Bathymodiolus childressi from two areas characterized by brine seepage and biogenic methane (GC425 and GC233) had very low δ15N (−3.7‰ and −16.6‰) and δ13C (−57.5‰ and −63.5‰) relative to areas with substantial thiotrophic production (GC234 and GC185). Bathymodiolus childressi from each chemosynthetic comm...