Nighttime multifrequency scintillation and 50-Mhz radar backscatter observations simultaneously performed over a nearly common ionospheric volume at the dip equator in Peru during March 1977 were used to study the relationship between the large-scale irregularities (-0.1-l km) giving rise to scintillations and small-scale irregularities (3 m) causing 50-MHz backscatter. It is shown that during the generation phase of equatorial irregularities in the evening hours, the kilometer- and meter-scale irregularities coexist, whereas in the later phase, approximately an hour after the onset, the meter-scale irregularities decay but the large-scale ones continue to retain their high spectral intensities. Futher, multistation scintillation observations from a host of geostationary satellites as well as from the Wideband satellite indicate that eastward-drifting irregularity structures detected around midnight cause significant scintillations at UHF and L band but generally fail to give rise to appreciable backscatter. Thus, contrary to expectations, it is possible to have even L band scintillations without any plume structure on backscatter maps. This indicates that at later local time a cutoff of the spectral intensity probably occurs at some scale length between 100 and 3 m. These observational results are discussed in the context of current theories of plasma instability in the equatorial ionosphere.