Temperature data obtained from June 1976 to May 1978 in the TRANSPAC ships-of-opportunity XBT program have been used to examine the large-scale evolution of synoptic thermal fronts from 165¿E to 135¿ W, from 30¿ to 50¿N in the mid-latitude North Pacific on a 2¿ latitude by 5¿ longitude grid. These results were referenced to a climatology computed from all XBT data collected in the region from 1968-1974. The primary indicator of frontal strength and positions was the zonal continuity of the modulus of horizontal temperature gradient in the upper 300 m. Over the 2-year period, the Subarctic Front was observed as a quasi-zonal coherent feature, ranging in latitude between 38¿ and 46¿, strongest at the sea surface, decreasing in strength toward the east. This front was found to have had two realizations, a northern one dominant in winter-spring between 42¿ and 46¿N and a southern one dominant in summer-fall between 38¿ and 42¿N. Generally, these frontal realizations were vertically coherent with depth. The transition from the southern front to the northern front occured in either Novemeber or December of both years, while the transition from the northern to the southern front took place in either March or April. In winter-spring, there existed considerable overlap of the existence of these two fronts; however in summer-fall, the northern front was not observed. The northern front had a tendency to be displaced southward 2¿--4¿ through the winter-spring period, while the southern front had no consistent tendency over the 2 years. At depth, the strength of these fronts shows no significant variability over the 2-year period, and the two fronts are comparable in strength. Two other thermal fronts were observed intermittently, i.e., the Subtropical Front in the eastern part of the region between 30¿ and 36¿ N and the Kuroshio Extension Front in the western part of the region along approximately 36¿N. These fronts were observed to form transient links or filaments with both realizations of the Subarctic Front.