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Immune cell characterization, immunological response and the associated gill oxidative balance were studied in the Patagonian freshwater mussel, Diplodon chilensis, using two microbiological immunostimulant models: Saccharomyces cerevisiae and Escherichia coli. Mussels were collected out of the breeding season in Paimún Lake and acclimated in the laboratory. Two exposure experiments were performed during two consecutive weeks: (1) mussels challenged with 500 yeast cells mL -1 ; and (2) mussels challenged with 1000 bacteria cells mL -1 . Microorganisms were added in the water every two days, alternating with 6000 lyophilized cells of the green algae Scenedesmus vacuolatus mL -1 . A control group, fed with S. vacuolatus, was set for each treatment. Morphological cell characterization was carried out in adherent hemocytes of D. chilensis hemolymph under control conditions. The most important cell type observed were the hyalinocytes (representing ca. 98% of the circulating cells), agranular cells with non-central polymorphic nucleus surrounded by cytoplasm; granulocytes (cells with cytoplasmic granules and non-central rounded nucleus) represented ca. 2%. Another two cell types were occasionally detected, binucleated hyalinocytes and hemoblast-like cells but were not considered for the analyses. Both adherent hyalinocytes and granulocytes exhibit phagocytic activity towards Congo red stained yeast, which was two-fold higher in granulocytes than in hyalinocytes, regardless of the applied challenge. Total hemocyte counts were diminished in mussels challenged with S. cerevisiae or E. coli. Hydrolytic and defense cellular enzyme activities were analyzed only for hyalinocytes. Both, S. cerevisiae and E. coli increased acid phosphatase activity. E. coli challenge diminished hemocyte lysosomal membrane stability and increased humoral phenoloxidase activity, while S. cerevisiae challenge did not affect any of these variables. Mussels challenged with E. coli showed increased gill antioxidant response without oxidative damage, while those challenged with S. cerevisiae showed no change in these variables., (Copyright © 2018. Published by Elsevier Inc.)