The Tasman Front represents a thermal boundary across the north Tasman Sea located between 31 and 35 degrees S over the Tasman Sea Ridge System in the south-west Pacific. Census data of planktic foraminiferal assemblages and oxygen isotopic variations of planktic and benthic foraminifera obtained from three DSDP sites (284, 590A and 588) were used to investigate the Pliocene history of the Tasman Front. The results indicate that at about 4.4 Ma the Tasman Front shifted to the south of 31 degrees S as reflected by a major change in the biogeographic distribution of planktic foraminifera. This change caused warming of the sea-surface, particularly in the area of DSDP site 590A. Temperature gradients between surface water and intermediate water (indicated by delta (super 18) O planktic-benthic) at DSDP590A also increased, suggesting that the site has been subsequently located beneath a more vertically stratified water column than prior to 4.4 Ma. The southward shifting of the Tasman Front might have been a response to the intensification of the East Australian Current, and the enlargement of the West Pacific Warm Pool in the western equatorial Pacific. Both of these oceanographic developments may have been caused by the curtailing of the westerly flowing South Equatorial Current due to tectonic closure of the New Guinea Seaway involving northward motion of the Australian Plate and an associated Pliocene orogeny in central New Guinea.