The Sargassum Frogfish (Histrio histrio), the only pelagic member of the frogfish family Antennariidae, is considered an obligate associate of floating mats of the brown algae Sargassum natans and S. fluitans (Adams 1960; Dooley 1972; Pietsch and Grobecker 1987). Between February and April 2010, 20 of these fish were observed in three mangrove-fringed bays in Virgin Islands Coral Reef National Monument, St. John, US Virgin Islands. All of them were clinging to clumps of the red alga Acanthophora spicifera growing on the submerged prop roots of red mangrove trees (Rhizophora mangle) distributed along an estimated total of 2,160 mof shoreline (Fig. 1). All of the fish were at a depth of less than 0.5 meters. Two individuals were seen on one prop root, but the other 18 were solitary. Their estimated standard lengths ranged from about 20 to 100 mm. Littler and Littler (2000, p. 295) published a photograph of one individual in blades of the green alga Ulva lactuca growing on a prop root in Belize. This is the first report of the Sargassum Frogfish living in association with attached algae.