Technical Abstract: Juvenile channel catfish were fed purified diets supplemented with magnesium (Mg) from Mg sulfate at levels of 0, 200, 400, 600, 800 and 1,000 mg/kg and 0, 200, 400, 600 and 800 mg/kg in two separate feeding studies. In study I, the effect of dietary levels of Mg on growth response, vertebral mineral content and macrophage chemotaxis were evaluated. Study II had similar objectives except that whole body mineral content was measured and resistance of channel catfish to Edwardsiella ictaluri challenge were also determined. Fish with an average weight of 10.89 g were stocked at a rate of 50 fish/110-L aquarium (study I). In study II, fish with an average weight of 4.14 g were stocked at rates of 40 fish/aquarium. Prior to stocking, each batch of fish were acclimated to the laboratory conditions and fed the basal diet for two weeks. The concentration of Mg in rearing water was 1.8 mg/L. Each diet was fed to fish in quadruplicate and triplicate aquaria to apparent satiation for 10 weeks for studies I and II, respectively. Fish fed the basal diet started to die as early as three days after the study began (17 days of feeding the diet without Mg supplementation). In both studies, weight gain, survival and feed efficiency were lowest for fish fed the basal diet but increased with increasing dietary levels of Mg. However, the differences between the values of each of these parameters for fish fed diets containing supplemental Mg were not always significant. Magnesium-deficiency signs observed were anorexia, sluggishness, convulsions, deformed snout, vertebral curvature, muscle flaccidity and high mortality. Vertebral and whole body ash concentrations were high but Mg content was low for fish fed the basal and the 200-mg Mg diets. Bone Ca content did not differ among fish fed different diets (study I), but whole body Ca tended to increase for fish fed the basal diet, suggesting the possibility of calcification of soft tissues. Macrophage chemotaxis in the presence of exoantigen was highest for fish fed diets supplemented with 400 and 200 mg Mg/kg for studies I and II, respectively. When expressed in terms of chemotaxis index, however, maximum or near maximum value was observed at a dietary Mg level of 400 mg/kg. Thus, a dietary level of Mg of 400 mg/kg from Mg sulfate was required for optimum growth and survival, maintaining high tissue levels of Mg, prevention of muscle flaccidity and skeletal deformity and stimulating macrophage chemotaxis. Dietary levels of Mg, however, had no effect on the resistance of juvenile channel catfish to E. ictaluri challenge.