<p>Freshwater ecosystems are increasingly exposed to environmental stressors capable of inducing physiological and immunological alterations in aquatic organisms. In this context, fish are widely recognized as valuable bioindicators because they reflect environmental changes at tissue and cellular levels. The present study aimed to characterize the morphology, morphometry, and NF-κB immunoreactivity of Rodlet cells (RCs) in the kidney and liver of the Neotropical teleost <i>Gymnotus carapo</i>. A total of 32 specimens collected from natural lentic environments (n = 14) and commercial holding facilities (n = 18) were examined using histological, histochemical (Periodic acid–Schiff, PAS), morphometric, and immunohistochemical techniques. RCs were identified in both organs and were classified into the four classical differentiation stages (granular, transitional, mature, and secretory), exhibiting the characteristic morphological features previously described for this cell type. Morphometric analysis demonstrated a progressive increase in cell length across the morphologically defined differentiation stages. Immunohistochemistry revealed NF-κB immunoreactivity in RCs located individually within the renal and hepatic parenchyma, associated with hematopoietic tissue and melanomacrophage centers, and occasionally within blood vessels. Immunoreactivity was detected in both the cytoplasm and the nucleus of RCs, consistent with the presence of NF-κB in these cells. Although the present study did not evaluate environmental contaminants or experimentally induced inflammatory conditions, the combined histological, morphometric, and immunohistochemical findings provide baseline evidence suggesting that RCs participate in immune-related processes and may represent potential histological biomarkers for future environmental monitoring studies. Nevertheless, this hypothesis requires further validation through controlled experimental approaches evaluating environmental stressors and inflammatory responses.</p>