Towed sensor data from the main thermocline in the Sargasso Sea south of Bermuda during the Mid-Ocean Dynamics Experiment are reported and compared with the Garrett and Munk internal wave model. The tows consisted of two sensor suites vertically spaced 11 or 21 m apart; the depth of tow was varied so as to continuously bracket an isothermal surface. Spectral analysis in the wave number band from 0.015 to 5 cycles km-1 results in a vetical displacement spectrum of the isopycnal surface that is statistically indistinguishable from earlier estimates in the same area. In this wave number band, agreement with both the 1972 and the 1975 versions of the Garrett and Munk internal wave model is good, though the model predicts a spectral level about one third too high. The vertical coherence spectra obtained from the temperature fluctuations at the two separations describe the horizontal wave number bands over which vertical coherency is lost. The 1975 model underestimates coherency but can be brought into better agreement by reducing the 'mode number scale'.