ABSTRACTEpulopiscium fishelsoni n. g., n. sp., is described from the gut of Acanthurus nigrofuscus, an herbivorous surgeonfish (Acanthuridae: Teleostei) from the Red Sea, Israel. The symbiont is placed in the kingdom Protoctista (or Protista) but more specific taxonomic assignment is impossible at this time. The organism is of an elongate oval shape, round in cross‐section, and highly mobile, with the ability to reverse direction rapidly. It lacks a mouth or other external structures, with the exception of fine surface filaments of an undetermined nature. Internally, there is a complex of folded membranes or tubules and a nucleus. No other organelles are evident. Reproduction is characterized by doubling of nuclei within the parental cell and emergence of daughter cells from a perforation in the parental cortex. Symbionts appear to exhibit circadian cycles of growth, reproductive activity, and movement within the gut. Although the organism greatly reduces local gut pH when present in large numbers, we know nothing of interactions between host and symbiont.