Langmuir oscillations excited by a powerful, high-frequency (HF) radio wave reflecting in the F-region have been studied in detail with the 430 MHz radar at Arecibo Observatory, Puerto Rico. Exceptionally good altitude and temporal resolution made it possible to closely examine transient plasma phenomena and allowed very weak plasma features to be seen in the ionosphere. The observations reveal an unexpected richness of complex plasma structure. Langmuir turbulence detected with the 430 MHz radar often occupies an extended altitude interval, particularly during the ''plasma line overshoot'' and at late times following HF turn-on. In the late time plasma environment, multiple layers of Langmuir turbulence are often observed. In general, important distinction can be made between the excitation of Langmuir oscillations in a ''cold'' background ionosphere and in an ionosphere ''preconditioned'' by prior HF transmissions. The greatest differences are evident in the altitude and range extent of the backscatter measured immediately after HF turn-on. ¿ American Geophysical Union 1990