This study tested the use of the essential oil of Zingiber officinale (EOZO) as an anesthetic for Astyanax lacustris and its possible histopathological effects on the gills. Two experiments were carried out. Experiment 1 determined anesthesia induction time and recovery time and the ventilatory frequency of animals of two size classes exposed to different concentrations of EOZO (0, 75, 150, 300, 450, and 600 μL L−1): Adults I (18.86 ± 4.81 g; 10.76 ± 0.87 cm) and Adults II (7.39 ± 2.57 g; 8.33 ± 0.86 cm). Ten fish were used for each size class and anesthetic concentration, distributed in a completely randomized design considering each animal as a replicate. Experiment II evaluated the gill histopathology of Adults II exposed to the same EOZO concentrations as used in Experiment I. Experiment I found concentrations of 300–600 μL L−1 EOZO and 150–600 μL L−1 EOZO to cause deep anesthesia for Adults I and Adults II, respectively. These recommended anesthetic concentrations provided induction and recovery times within acceptable limits for fish. Ventilatory frequency was reduced in the minimum concentrations indicated for each size class. No mortality was observed within 24 h post-test. Experiment II found irreversible damage to gill tissue for fish exposed above 300 μL L−1 EOZO. Therefore, EOZO concentrations of 300 μL L−1 and 150 μL L−1 are recommended for Adults I and Adults II, respectively.