Understanding the role of thermohaline circulation in past climate depends on proxy-based reconstructions of deep-water mass composition. A growing body of data indicates that the neodymium (Nd) isotopic composition of fossil fish debris found in deep-sea sediments can be used as a gauge of deep-water mass composition. This paper reviews the basis for the use of fossil fish debris Nd isotopic values as a proxy for ancient deep-water mass composition and then presents an example of how ancient thermohaline circulation patterns are reconstructed using records of fish debris Nd from deep-sea sediment cores. Fish teeth and debris acquire enhanced Nd concentrations ( approximately 100s to approximately 1000 ppm) during an early diagenetic reaction at the sediment/water interface. Data published in 2004 by Martin and Scher and presented in this work confirm the assertion that fish teeth and debris record and retain a deep-water mass signal. The widespread stratigraphic and geographic occurrence of fish remains in deep-sea sediments enables construction of much higher-resolution records than previously afforded by Nd isotopic records derived from Fe-Mn crusts. The utility of fish debris Nd isotopic records is apparent when applied to the late Paleocene-early Eocene interval ca. 55 Ma, an interval of time poorly represented in Fe-Mn crust records. Fish debris records from a suite of deep-sea sedimentary sections indicate a mode of thermohaline circulation fundamentally different from the modern-deep waters formed primarily in the Southern Ocean, with no source of deep-water formation in the North Atlantic. This work reinforces the validity and effectiveness of fish debris Nd isotopic records as a tool for reconstructing ancient thermohaline circulation patterns.