Abstract: The southern catfish (Silurus meridionalis Chen) were fed with artificial diets in a recirculated filtered rearing system maintained at 27.5℃ (± 0.2℃) for 6 weeks.Using the chromic oxide as an indicator, nutrient apparent digestibilities of six isoenergetic (20kJ/ g) diets containing 35%- 60% protein at 5% increments were determined With the increase from 39.86% to 55.73% in dietary protein level, the apparent digestibilities of dry matter (Ddm), protein (Dp), lipid (D1), energy (De) and nitrogen free extract (Dnfe) increased from 36.68%, 77.13%, 77.13%, 71.43% and 55.94% to 82.46%, 94.22%, 94.96%, 93.56%, and 84.96%, respectively (p 0.05).The analysis of linear regression indicated that Ddm, Dp, D1, De, and Dnfe were correlated positively with protein level, negatively with nitrogen free extract level.It is suggested that the correlation of apparent digestibility of protein in the fish with dietary protein level alight come from three sources as follows: a) True digestibility of protein which decreases with the decrease of protein level.b) Endogenous nitrogen in faeces which may change with the protein level of diet c) The two mechanisms occurred simultaneously.It is argued that the hypothesis by Ogino and Chen (1973) as to the relationship between protein digesitibility and protein level in diet should be invalid.