A series of oblique hauls with Bongo nets (0–1000 m) was made during the period of August 2002 through August 2004 in the Oyashio region, western subarctic Pacific, to investigate abundance, biomass and life cycle patterns of the three predominant euphausiids (Euphausia pacifica, Thysanoessa inspinata and T. longipes). While the three euphausiids occurred throughout the entire study period, E. pacifica was the most abundant (1,120 indiv.m⁻², or 832 mg C m⁻²), followed by T. inspinata (163 indiv.m⁻², or 144 mg C m⁻²) and T. longipes (73 indiv.m⁻², or 75 mg C m⁻²). Judging from the occurrence of females with spermatophores and furcilia larvae, the spawning was considered to take place twice a year (April–May and August) for E. pacifica, year-round (peak season: March–May) for T. inspinata and in spring (March–May) for T. longipes. The population structure in terms of size (=total length) frequency distributions of the three euphausiids was characterized by the frequent co-occurrence of 2–3 cohorts in the same samples. The maximum size of males and females found were 21 mm and 24 mm, respectively, for E. pacifica, 18 mm and 23 mm, respectively, for T. inspinata, 27 mm and 31 mm, respectively, for T. longipes. Tracing the sequence of cohorts, the life spans of E. pacifica, T. inspinata and T. longipes were estimated to be 17–26 months, 17–19 months and 29–31 months, respectively. These results are compared with reports of the same species in other habitats in the light of regional variations.