Food habits of threadfin hakeling Laemonema longipes, the dominant demersal fish in the upper continental slope of the western sub-Arctic North Pacific, were described based upon >1400 specimens collected off north-eastern Honshu Island and south-eastern Hokkaido Island, Japan. Threadfin hakeling depended exclusively on pelagic organisms including copepods Neocalanus cristatus, Euphausia pacifica, shrimps Sergestes similis, firefly squid Watasenia scintillans and myctophids Diaphus theta. Threadfin hakeling showed a clear size-dependent dietary variation, with mesozooplankton and shrimps being important for small- and medium-sized fish (≤400 mm standard length (SL)), and myctophids being important for large-sized fish (>400 mm SL). Since all of these prey undertake diurnal vertical migration, it was concluded that threadfin hakeling take an important role in capturing energy and materials transported from the epipelagic zone to the bathypelagic layer over the upper continental slope. Dietary variation among different areas, seasons and depths were also examined. Of these, areal difference was rather obscure reflecting common prey field in the Tohoku and Doto areas under the prevalence of the North Pacific Intermediate Water, whereas clear seasonal difference was found in the Doto area; the 0-group of walleye pollock Theragra chalcogramma was ingested only in the late summer and early autumn in this area.