Quasi-sinusoidal density waves were frequently observed during the end of the Pioneer Venus Orbiter (PVO) mission when the orbiter was at low periapsis (Brace, this issue). These waves occur at altitudes ~145--155 km and have wavelengths ~1 km. It is suggested that a radial, ambipolar electric field E0, directed downward, is established in the Venus ionosphere during electron pressure enhancements above ~160 km. This field generates an electron E¿B drift VE; the ions move radially and do not E¿B drift because they are unmagnetized (i.e., &ngr;in≫&OHgr;i). This drift is shown to drive a collisional drift wave instability for sufficiently large values of VE, nominally, VE>vi where vi is the ion thermal velocity. For parameters typical of the nightside Venus ionosphere, this instability generates plasma fluctuations with wavelengths ~1 km, consistent with observations.