The reproductive ecology of shark catfish Helicophagus waandersii in the Mun River, the largest Mekong tributary in Thailand, was studied from July 2003 to December 2004. The aim of the study was to determine whether shark catfish, which contribute greatly to fish catches, benefits in terms of spawning, from the management regime of the Pak Mun Dam to open all sluice gates annually for 4 months during the rainy season. In total, 228 females were studied. The highest average gonadosomatic index (4.42% AE 1.90%) was observed in the rainy season in June 2004. Fecundity ranged from 21 550 to 191 540 eggs. Three hormonal profiles viz. testosterone (T), 17 b-estradiol (E 2 ), and 17, 20 b-dihydroxy-4-pregnen-3-one (17, 20 bP) were determined by radioimmunoassay. The highest levels of T (0.96 ng ml À1 ), E 2 (14.37 pg ml À1 ), and 17, 20 bP (0.81 ng ml À1) were respectively measured in April to May, July, and May. Changes in the maturity stage and the annual sex hormonal profiles revealed that the spawning period of H. waandersii is from May to June. There was no spatial statistical difference (p > 0.05) for each sex steroid profiles. The results indicated that H. waandersii spawn during the rainy season in the rapid areas, both downstream and upstream, and the sluice gates opening management regime for 4 months in the rainy season is of advantage for H. waandersii in terms of feeding and nursery grounds in the upstream area.