The nematode Hysterothylacium sp. (Nematoda, Anisakidae) may cause significant economic losses in fish production. In order to control the nematode larvae in juvenile pirarucu (52 ± 3.2 g), the efficacy of essential oil (EO) of Piper aduncum when added to feed was evaluated. Chemical analysis of the EO composition had been performed previously, showing that 92% of the substance was dillapiole. After the palatability test, the efficacy of the EO was evaluated (0, 32, 48, 56, and 64 mL/kg) at 7 and 15 days, based on the blood parameters and the parasite load of the fish. The results showed significant differences for hematocrit and corpuscular constants at 7 days and for total proteins at 15 days between the treatment groups and the control group (without EO). The parasitic indices showed a prevalence of 100% for stage L3 larva of the nematode Hysterothylacium sp. At 7 and 15 days, the values of average intensity, average abundance, and intensity of parasites decreased with increasing EO concentration, and at 15 days, these were significantly lower between the treatment group and the control group, with an efficacy of 76.21% for 64 mL/kg. Thus, the EO of P. aduncum demonstrates anthelmintic potential in the control of nematode larvae in pirarucu without affecting fish homeostasis.