Bathymodiolus childressi is a foundation species at methane seeps on the upper-continental slope of the Gulf of Mexico. Other species of the genus are known to gain the advantage of variable food availability through trophic plasticity, by hosting dual microbial symbionts while retaining their own particle feeding ability. B. childressi, however, hosts only a single methanotrophic symbiont, and the possibility of trophic plasticity has not been fully examined in this species. Feeding strategies of archival specimens from 2 geochemically contrasting seeps from the Gulf of Mexico (Bush Hill and Brine Pool NR-1) were characterized using 4-source mixing analysis of δ13C, δ15N and δ34S values. Bush Hill mussels used a single thermogenic methane pool and derived N and S from different sources. Brine Pool mussels used 2 separate methane pools; the primary one being biogenic and the secondary possibly a mix of biogenic and thermogenic. Utilization of particulate material was less common at Brine Pool than at Bush Hill. Bush Hill appears to offer lower levels of methane-based resources with particulate material having a greater, and sometimes dominant role in nutrition. Spatial patterns within the seeps were found but were not reflective of simple gradients. Some temporal changes occurred at both yearly scales and between samples, which were collected 17 yr apart. The 4-source mixing model used extrapolations of mussel isotope values and limited environment characterization to infer likely trophic sources. The actual sources, however, remain unidentified. Future research across a wider range of seeps as well as experimental studies should be used to test the validity of the model.