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We investigated the effects of low dietary protein in isocaloric diets on sex reversal of Monopterus albus by evaluating the oestradiol (E2) and testosterone (T) concentrations, gonadosomatic index (GSI), sex ratio and gonad structure at the histological level. Fish (9.50 ± 1.50 g average initial weight; n=3 per group) were fed with five practical diets containing 100, 150, 200, 250 or 400 g kg−1 crude protein to apparent satiation for 15 months. Serum E2 and T concentrations were determined by radioimmunoassays. E2 concentrations and GSI significantly increased while T concentrations decreased as the dietary protein level was raised. Fish fed 400 g kg−1 of dietary protein had significantly higher E2 concentrations and GSI than those fed with lower dietary protein levels. The T concentrations of fish fed 100 g kg−1 of dietary protein was significantly higher than that of fish fed higher dietary protein levels. The shift of sex ratio towards more male and intersex fish was observed with decreasing dietary protein levels. Therefore, low dietary protein level may promote sex change from female to male in M. albus. This study provides important information for successful reproductive management and may be exploited for aquaculture of this species. [ABSTRACT FROM AUTHOR]