The effect of heavy river flood during 1987/88 on fish population was studied in Khor al-Zubair, northwestern Arabian Gulf. The fish structure in the Khor changed dramatically due to the intrusion of freshwater species. Six freshwater species (Heteropneustus fossilus, Liza abu, Alburnus capito, Barbus luteus, Parasilurus triostegus, Cyprinus carpio) comprised 38.8% during the heavy flood at the northern part of the Khor (Junction station), while only the first five species contributed 17.8% the total number at the middle of the Khor (Port station).