<jats:title>Abstract</jats:title> <jats:p>Kobari, T., Moku, M., and Takahashi, K. 2008. Seasonal appearance of expatriated boreal copepods in the Oyashio–Kuroshio mixed region. – ICES Journal of Marine Science, 65: 469–476. Seasonal changes in abundance, biomass, and community structure of ontogenetically migrating copepods were investigated using monthly zooplankton samples from the Oyashio–Kuroshio mixed region (OKMR) to evaluate impacts of the copepod community expatriated from more northerly waters on carbon flow in southern areas. The copepod community comprised more than half the total zooplankton biomass and exhibited seasonal fluctuations, although they were minor components in terms of abundance. The ontogenetic migrants increased their abundance and biomass during May–July when Oyashio waters dominated at the surface. They were almost absent from 0 to 150 m during August to December. Predominant species were Eucalanus bungii and Metridia pacifica for abundance and Neocalanus cristatus for biomass. Seasonal changes in their abundance and stage composition indicated that surface development, dormancy, and reproduction were comparable in schedule with those reported in the Oyashio region. We suggest that the ontogenetically migrating copepods appearing in this area are populations transported with a southward Oyashio intrusion during surface development seasons, then by both downward migration and advection of dormant populations in the submerging Oyashio flow. We discuss impacts of the expatriated copepods on food availability for mesopelagic fish in the OKMR.</jats:p>