The surface duct is the mixed-layer underwater sound channel generally bounded by the ocean surface and the thermocline. In a recent experiment, abundant measurements of surface-duct propagation loss were made concurrently with detailed environmental sampling. The experiment requiring three ships was designed to fill a gap in the knowledge of surface-duct propagation at frequencies below 8 kHz that has not been addressed. Propagation loss was measured at frequencies from 0.4 to 20.0 kHz, at depths from 20 to 600 ft, over ranges of 0–18 miles. The experiment was novel because for the first time adequate environmental measurements were made over the entire acoustic field. Three different instruments measured sea surface roughness. Sound speed profiles were continuously sampled both in space and time: every 50 ft along the track by a towed thermistor chain, every 10 sec by a string of thermistor buoys spaced along the propagation path. The accuracy of these systems was ±0.02°C or ±0.06 m/sec in terms of sound speed. The purpose of the experiment was to provide data required for evaluating several theories of surface duct propagation loss.