We report the presence of Mesorhizobium, a genus best known for its nitrogen-fixing symbiosis with terrestrial legumes, associated with the marine polychaete Meganerilla bactericola (Annelida: Nerillidae). Abundant epibionts were previously described as coating the exterior of M. bactericola, which is found within the anoxic sulfide-oxidizing microbial mats of the Santa Barbara Basin, California, USA. 16S rRNA investigation of the bacterial community associated with this polychaete discovered the presence of bacteria belonging to Mesorhizobium. We identified these bacteria using phylogenetic analyses of 16S rRNA and three additional functional genes, nifH, atpD and recA, and group-specific fluorescence in situ hybridization (FISH).