A permeameter was designed and constructed, and its theory of operation was developed; it has a subsurface probe for internal permeability measurements in poorly consolidated sediments. It was used to measure the permeability of sand at five depths in the upper 1.3 m (4.3 ft) of structural features on a large vegetated composite crescentic dune in the Nebraska Sand Hills. Permeability measurements were taken at 34 locations on the dune's steep leeward face, its long windward slope, fans below the leeward face, blowouts, peaks, crests, transverse ridges, benches, and the heads of gullies. Correlation of the permeabilities with the morphology of the dune is illustrated by a plot of the depth-averaged permeabilities and dune profile along a transect perpendicular to the lee face. Some of the permeabilities do not match the values anticipated on the basis of the theory of sedimentation (and experimental results) for active crescentic dunes. Although the large-scale features of the dune studied were evidently because of the original eolian sedimentation, the permeability was found to be strongly influenced by dune degradation processes involving running water and animal activity.