A synoptic expendable bathythermograph survey of the warm‐core Gulf Stream ring 81‐D, conducted in September 1981 when the ring was located 550 km south of Halifax, Nova Scotia, provided the basis for a detailed study of the ring’s sound‐speed field. Salinities used for computing sound speed were derived from nonsynoptic conductivity–temperature–depth data collected during the survey. Analysis of the sound‐speed field, using vertical and horizontal sections and satellite imagery, revealed the ring’s typical high sound‐speed core, but also showed increased horizontal and vertical gradients along the ring’s western margin as compared to its eastern side. This asymmetry appears to be related to a surface streamer of warm, higher sound‐speed Gulf Stream water overlying colder, lower sound‐speed slope water, resulting in a subsurface sound duct centered 100 m below the surface along the ring’s western margin. The subsurface duct and increased vertical gradient resulted in 30–50 dB greater loss at depths of 100 and 200 m in the ring’s eastern margin as compared to its western side and center. Strong ray channeling and refraction of a 2000‐Hz source at a depth of 100 m was also noted. Based on these observations and predictions, warm‐core rings may possess regions of increased acoustic range dependence due to the interactions of waters along their margins.