Tambaqui (Colossoma macropomum) is among the most cultivated fish species in tropical countries. Stress is the main cause of disease in fish farms. The genus Aeromonas is a common causative agent of fish diseases. This work reports the identification of Aeromonas species colonizing gills of C. macropomum submitted or not to a confinement stress. We also evaluated changes in serum levels of lectins (carbohydrate-binding proteins that are components of fish immune system) in tambaqui submitted to a challenge using two isolated Aeromonas strains. Gill tissues from stressed and unstressed fishes were used to isolate Aeromonas. Then 72 Aeromonas strains were isolated, 97% being from stressed fishes. Among these, 63 were identified at species level and 6 were classified as atypical Aeromonas strains. The most prevalent species were Aeromonas bestiarum and Aeromonas caviae and their strains were used in bacterial challenges. The lectin serum levels significantly increased after 24 h of infection with A. bestiarum; however, no significant increase was found for infection with A. caviae. In conclusion, C. macropomum gills are susceptible to colonization by different Aeromonas species, mainly at confinement stressful conditions, and serum lectins may have a role in the acute immunological response towards infection by A. bestiarum.