The first occurrence of the planktonic foraminiferal species Neogloboquadrina acostaensis,which defines the zonal boundary between zones N.15 and N.16, was precisely determined just above the KT-1 Tuff in the lower part of the Kubota Formation in the Tanagura area, Northeast Japan. According to radiometric ages of the KT-1 Tuff, this biohorizon is estimated at 10.6 Ma. Compiling deep-sea drilling data demonstrates that the biohorizon as synchronous at least in the mid-latitudes.