We present new experimental findings that provide insight into the nature of 150-km echoes. A finding, using data obtained with a 49.92-MHz radar on Pohnpei (6.96 ¿N, 158.19 ¿E, 0.5¿ magnetic dip angle), is an unusual east-west asymmetry in which the altitude profile of echo occurrence depends on viewing direction. The other is the narrowness of Doppler spectral widths associated with these echoes. When considered with other known properties of 150-km echoes, a scenario for 150-km echoes emerges in which 3-m-scale field-aligned irregularities (FAI) are imbedded in tilted, sheetlike structures in plasma density. These meter-scale FAI are envisioned to have narrow Doppler spectral widths if excited directly by a linear plasma instability, and they would consist of a narrow angular spectrum of plane plasma waves if the instability is weak. The latter could produce the observed east-west viewing asymmetry. ¿ 2000 American Geophysical Union