Distributions of phytoplankton and tintinnid species were examined along a transverse line crossing an ocean front in the Sanriku Waters, which had been formed about 60 miles off northeast Japan. The distribution patterns of these microplankton were analysed by referring to distrbution of pollen grains simultaneously examined. The results showed that there was a fairly sharp segregation between warm water species (e. g. Asteromphalus hookerii, Ceratium fusus, etc.) and cold water species (e. g. Chaetoceros concavicorne, Acanthostomella norvegica, etc.). While some cosmopolites (e. g. Dinophysis fortii, D. infundibulus, Eutintinnus lususundae, etc.) occurred commonly over the entire transverse line, some coastal species (e. g. Nitzschia seriata, Thalassiosira sp., Helicostomella sp., etc.) were abundant within the mid-frontal zone. The latter species might be of neritic origins and seemed to be transported from coastal areas into the front by a jet stream predominating at the boundary zone.