Thermal fronts in the North Pacific from June 1976 to May 1980 have been mapped at the 300-m level by using temperature data obtained in the region 30¿--45¿N, 160¿E--160¿W in the TRANSPAC XBT Volunteer Observing Ship Program. The origins of the paths of the major frontal bands in the study region (i.e., the Kuroshio Extension, the Subarctic Fronts, and the Subtropical Front) appear to be related to the principal bathymetric features of the western mid-latitude North Pacific (i.e., the Shatsky Rise, the Emperor Seamont Chain, and the Hess Rise). In the long-term mean, the Kuroshio Extension Front at 35¿N was observed to have bifurcated at the Shatsky Rise near 160¿E into two bands. One band continued northeast along the Shatsky Rise to 42¿N to become the North Subarctic Front. The other band continued zonally eastward where it encountered the Emperor Seamounts, near 36¿N, 170¿E, and trifurcated into bands which tracked through passages between the Seamounts near 37¿N, 34¿N, and 32¿N, respectively. Downstream, the most northern of these three bands became the South Subaractic Front near 39¿N, the middle band continued as the Kuroshio Extension Front at 35¿N, and the most sourthern band became the Subtropical Front near 32¿N. The middle band tracked north of the Hess Rise, while the most sourthern band tracked south of the Hess Rise. A near-meridional frontal band linked the North and South Subarctic fronts in the Emperor Seamount region near 170¿E. In the annual mean and individual seasonal mean maps, the essential character of topographic influence of the long-term mean was observed. However, the individual frontal bands varied seasonally about the long-term mean by ¿2¿ of latitude. Also, all four major frontal bands were not always observed, and intermittent linkages and additional filaments also occurred.