Abstract: The present study was conducted to compare the utilization of dietary crystalline L-lysine hydrochloride (Lys-HCl), crystalline L-lysine sulphate (Lys-H2SO4) and microcapsulated L-lysine hydrochloride (microcapsulated Lys) by the channel catfish (Ictalurus punctatus). Five diets with iso-nitrogen and iso-energy were designed including the positive control (5% fish meal, 15% soybean meal), the negative control (2.5% fish meal, 0 soybean meal). Crystalline Lys-HCl (Lys-HCl diet), crystalline Lys-H2SO4 (Lys-H2SO4 diet) and microcapsulated Lys-HCl (microcapsu-lated Lys diet) were added into the negative control to from the other 3 diets with the same level of Lys as in the positive control. We then gave the five diets to the channel catfish with an initial body weight of (54.40.1) g for 60d, in order to investigate their effects on the growth performance, the serum biochemical indices and the activities of the protein digestive enzymes. Compared to the negative control, the addition of dietary crystalline Lys-HCl or Lys-H2SO4 did not generate significant difference on the growth performance (P0.05). However, fish fed with microcapsulated Lys showed a higher weight gain rate (WGR) (+20.7%) and a lower feed conversion ratio (FCR) (-16.0%) (P0.05) than those fed with negative control diet, and their WGR and FCR were close to the positive control group. Compared to the negative control, Lys-HCl, Lys-H2SO4 or microcapsulated Lys did not significantly affected the activities of serum alanine transaminase (ALT), aspartate transaminase (AST), alkaline phosphatase (ALP) and intestine protease (P0.05), but did increase the activity of protease in the stomach (P0.05). Dietary microcapsulated Lys also significantly increased the activity of protease in the hepatopancreas. These results suggested that adding microcapsulated Lys but not crystalline Lys-HCl or Lys-H2SO4 into the low-Lys diet could improve the growth performance of the channel catfish.