The enzymatic activity of aerobic and anaerobic metabolic pathways in Hyalinoecia artifex, a polychaete inhabiting the deep ocean, is reported. In addition, the allometry of its anaerobic and aerobic enzymatic activity is analysed. The aerobic metabolism was measured using the electron transport system activity technique (ETS), whereas the anaerobic metabolism was estimated using the activity of lactate dehydrogenase (LDH), octopine dehydrogenase (OPDH), alanopine dehydrogenase (ALPDH), strombine dehydrogenase (STRDH), and ethanol dehydrogenase (EtOHDH). The ETS activity was about 296.18 (µLO2 h−1 g−1), which is within the range described for polychaetes and other benthic metazoans. The anaerobic enzymatic activity expressed as µmol NADH min−1 g−1 was: LDH = 0.35, OPDH = 0.11, ALPDH = 12.66, STRDH = 10.78 and SDH = 0.48. The slope of the allometric relationship between specific aerobic metabolism and body size was −0.35. In the case of the allometric scaling of the anaerobic metabolism, only LDH presented a significant relationship, with a slope of b = 0.44. This positive scaling is consistent with the pattern emerging from the scarce literature on the allometry of anaerobic metabolism in marine biota.