Hypostomus fluviatilis (Schubart, 1964) is an armoured catfish (Loricariidae) found in the Mogi-Guaçu river, State of São Paulo, Brazil. It is a species which may interest some fish culturists, due to is excellent meat. A total of 47 specimens (26 males and 21 females), caught from March to September 1975, at Emas Fali, Mogi-Guaçu river, was analyzed. Some meristic characters, together with biological aspects, are presented.