Small-scale (≤1 km) plasma irregularities have previously been observed in situ from the E region to an altitude of 8000 km. In this paper we report results from the Viking plasma wave experiments which extends the measurements of high-latitude irregularities in two ways: (1) we have acquired electron density fluctuation measurements up to an altitude of 13,500 km and (2) for the first time a measurement techniques was used that made a phase velocity deduction possible from in situ measurements. The spacecraft was equipped with two spatially separated Langmuir probes, each with an ability to measure relative probe current fluctuations with frequencies from dc to about 400 Hz. Under certain assumptions the current fluctuations could be interpreted as relative plasma density fluctuations, Δne/ne. Data from this ''interferometric'' instrument has been used to infer the distribution and nature of plasma irregularities along Viking orbits. It is demonstrated that the interferometric technique offers great advantages compared to single point measurements in this kind of study. It is shown that the observed small-scale plasma irregularities are ''nondispersive'' and convecting with the background plasma. They exhibit a power law frequency spectrum as observed in the satellite reference frame. The spectral index varies with location. An attempt to map source regions by identifying regions of high power and shallow spectrums is made. It is concluded that plasma irregularities, in accordance with our selection criteria, can be present in all regions of space encountered by Viking. ¿ American Geophysical Union 1990