During a Naval exercise in November 1975, R/V Chain ran a line of expendable bathythermographs (XBT's) three times across the edge of a warm anticyclonic Gulf Stream eddy (i.e., across the eddy front). The first two lines were 200 km long, spaced about 24 hours apart, and also extended across the Gulf Stream north wall. The third line was 90 km long and occupied nearly 10 days later. Sound speeds were calculated by using the equation of Wilson (1960) from XBT temperatures and a salinity field derived from exercise data. Detailed sound speed analyses clearly delineated the eddy edge and indicated that anticyclonic eddy fronts are among the strongest fronts found in the North Atlantic Ocean. Comparison of the three lines showed that the eddy front advanced to the west at an average rate of 9 km/day (0.2kn) over the course of the 3-week exercise. A period of rapid frontal migration followed and apparently was caused by high winds from a major storm halfway through the exercise. Preliminary analysis of acoustic data indicates signal enhancements of several decibels for a source outside the eddy and a receiver within the eddy.