The lack of studies integrating biostratigraphic and paleoceanographic analysis of pre-Pleistocene sediments in the Gulf of Mexico provides the purpose for this research. The pre-Pleistocene section (98-228.5m) of Hole 625B (ODP Leg 100) located in the Northeast Gulf of Mexico was analyzed at 1m intervals. Approximately 300 planktonic foraminifera were identified from each sample and then placed into operational taxonomic groups based on presumed similarities in water mass and depth preferences. Paleoceanographic/paleoclimatic data was obtained using 1) planktonic/benthic ratios, 2) relative numbers of fragments, 3) carbonate coarse (>63 micrometers) fraction percentages, 4) sediment accumulation rates, 5) diversity indices, 6) percent abundances of operational taxonomic groups, and 7) polytopic vector analysis. Important datums include the first appearance (FAD) of Globorotalia margarita, the last appearances (LADS) of Globigerina nepenthes, Globorotalia margaritae, Dentoglobigerina altispira altispira, and Globorotalia multicamerata and the first appearance of Globorotalia truncatulinoides. The planktonic foraminiferal data suggests that an unconformity with a hiatus of at least 500, 000 years separates middle and upper Pliocene sediments. Paleoceanographic/paleoclimatic analysis results in the delineation of three intervals. The latest Miocene-early Pliocene is interpreted as having been influenced by the deep, cold, corrosive, East Pacific waters prior to the emergence of the Isthmus of Panama. A change in the paleoceanographic conditions 3.9 Ma (early-middle Pliocene boundary) is evidenced by a decrease in the abundance of the Neogloboquadrina plexus and an increase in the abundance of the Globigerinoides ruber group. During the middle Pliocene, following the emergence of the Isthmus of Panama, the waters came into equilibrium and became well-stratified resulting in the exploitation of numerous niches in the water column as evidenced by increased diversity. The interval ends with an unconformity which most likely represents the onset of northern hemisphere glaciation. The late Pliocene is characterized by fluctuating surface water temperatures and non-corrosive bottom waters.