An 18-week feeding trial was carried out under controlled conditions to compare the effects of L-carnitine at six levels (10, 150, 350, 650, 950 and 1250 mg kg-1) on the growth performance, nutritional efficiency indices, hemolymph indices and finally, biological responses against 12-hr air exposure challenge of juvenile (7.58±0.39 g) crayfish Astacus leptodactylus leptodactylus. The significantly (p<0.05) highest values of final weight (63.58 g) and SGR (1.67% day-1) and the lowest FCR (2.11) were observed in the juvenile crayfish fed the diet containing 650 mg kg-1 L-carnitine. With an increment in the dietary L-carnitine levels, VFI increased significantly (p<0.05) from 2.28 to 3.66 % BW day-1. The crayfish fed the levels of dietary L-carnitine higher than 350 mg kg-1 showed the significantly (p<0.05) higher values of PER and PPV compared with those of fed 10, 150 and 350 mg kg-1. The significantly (p<0.05) lowest LPV was observed in crayfish fed the diets containing 650 and 950 mg L-carnitine kg-1. The juvenile crayfish fed the diet containing 650 mg kg-1 L-carnitine had the significantly (p<0.05) highest THC (105.36 ×105 cell ml-1), PO (7.03 U min-1), SOD (4.83 U min-1), GSH (3.53 mg g-1 protein), GPX (4.23 mg g-1 protein) and TBARS (4.40 mg g-1 protein). There was an inverse trend between SOD and TBARS. After 12-h air exposure challenge, the juvenile crayfish fed the diet containing 650 mg kg-1 L-carnitine showed significantly (p<0.05) highest activities of THC, PO, SOD, GSH and GPX. The dietary L-carnitine level of 650-950 mg kg-1 could improve growth performance, feed utilization and antioxidant defence system. Polynomial regression of SGR, FCR, PPV, PER and LPV suggested that the optimum dietary L-carnitine level could be higher than 350 and <950 mg kg-1 in crayfish reared in culture conditions.

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