The aim of the work is to study the leukocyte composition in blood of deep-water fish species by the example of Antarctic toothfish Dissostichus mawsoni. Fish were caught in January–February 2012 in the Ross Sea (Statistical Subarea 88.1) at depths from 690 to 1183 m from onboard “Yantar-31” longline fishing vessel. Blood samples were collected from the caudal artery of immature fish (10 specimens) with average mass 21.4 kg and length 114 cm. The composition of leukocytes in Antarctic toothfish is similar to the leukocyte composition in freshwater fish species except basophils (Ivanova, 1983; Golovina and Trombitskii, 1989). Lymphocytes constitute the main portion (84.8 ± 2.13%) in the leukocyte formula; eosinophils (6.0 ± 1.29%), neutrophils (4.8 ± 0.92%), blast cells (2.7 ± 0.42%) and monocytes (1.7 ± 0.33%) are less numerous. Among studied Antarctic species, Antarctic blackfin icefish Chaenocephalus aceratus (Barber et al., 1981) have a similar composition of leukocytes.