Demonstration of animal-bacterial symbioses fueled by methane in two annelid species (a serpulid Laminatubus and sabellid Bispira) and Methylococcales bacteria. Samples were obtained from deep-sea methane seeps along the Pacific continental margin of northern central Costa Rica. These worms represent a new addition to the animals associated with methane-oxidizing bacteria, and indicate additional consumers of methane in the deep sea.