The aim of the study was to investigate the effect of garlic extract on hematological, immune and metabolic systems of Rhamdia quelen juveniles and the animals’s resistance to Ichthyophthirius multifiliis. The fishes were fed twice daily during 77 days with diets containing 0%, 0.5%, 2.5% and 5% of garlic extract. After that, half of the animals of each treatment were submitted to the challenge with the parasite I. multifiliis, maintaining the diets. The other half of the animals were maintained in the original system receiving the dietary supplementation. The use of garlic extract in the diet did not promote significant changes in glycaemia, hepatic glycogen content, tissue and plasma aminotransferase activities, and tissue lipid peroxidation levels. On the other hand, it stimulated the synthesis of muscle glycogen and promoted a reduction of hepatic catalase activity. In terms of hematological and immune systems, the use of garlic extract increased the number of red blood cells, but reduced hemoglobin content. Furthermore, it was not able to change the total number of leukocytes and thrombocytes, but it promoted an increase in the number of basophils and reduced the monocyte count of the animals. The dietary supplementation with garlic extract did not act as a growth promoter for silver catfish juveniles, did not influence the basal metabolic homeostasis, immune or hematological responses of the fishes and did not improve the resistance to the infestation with the I. multifiliis in the concentrations and periods studied