Epulopiscium fishelsoni, or "epulo" for short, is a species of Gram-positive bacteria that have a symbiotic relationship with surgeonfish. These bacteria are most well known for their large size, ranging from 200–700 µm in length. Until the discovery of Thiomargarita namibiensis in 1999, these were the largest bacteria known. The bacterium has not been grown in the lab, but scientists have gained a better understanding of it through microscopic analysis.