A study of the nocturnal F layer over Arecibo, Puerto Rico, reveals that substantial spatial variations in the height of the layer are frequently present. The variations are deduced from electron density profiles measured by the incoherent radar in which the radar beam is offset 17¿ in zenith and rotated continuously in azimuth. This scanning procedure shows that the ionosphere above Arecibo often has well-defined bands where the F region as a whole is alternately high and low. The transition between adjoining regions is very sharp. For example, electron density profiles taken at points only a few degrees apart in azimuth, which corresponds to a spatial separation of less than 10 km, can show variations in hmax of up to 70 km. It is suggested that these bands are manifestations of the so-called 'Perkins' instability.