Starting from 0415 UT on 8 January 2000, ROCSAT-1 observed intermediate-scale (0.1 to 50 km) ion density irregularities in conjunction with mesoscale (∼50 to 1000 km) density and flow undulations in a total of eight consecutive orbits from low to middle latitudes in the northern hemisphere of nightside topside ionosphere. The mesoscale undulation indicates in-phase variation between the outward as well as westward flow and the density enhancement. The induced electric fields derived from the flow perturbations point along the northeast-southwest direction. The locations of the disturbances mapped from the onset of each mesoscale undulation event observed in eight consecutive ROCSAT orbits form a frontal structure that is aligned from northwest to southeast. These disturbances cover a large geographic area from 120° to 300° in longitude and from latitude 20° up to the ROCSAT turn-around latitude at 35° in the nightside ionosphere. Since the first onset of mesoscale undulations is preceded by a sudden enhancement of the background flow in the southeast direction, which can be induced by a southwest-pointed electric field, the cause of mesoscale undulations is identified as the Perkins instability. On the other hand, it is difficult to explain the occurrence of intermediate-scale irregularities that either appear alone in time and space, or coexist with mesoscale undulations. The signature of intermediate-scale irregularities, in addition to the density fluctuations, is the existence of a dominant zonal flow fluctuation that is completely different from the characteristics of mesoscale undulations. One possible explanation for the existence of intermediate-scale irregularities is from the mapping of irregularity structures from E region or lower F region. However, theoretical model for the production and mapping process of intermediate-scale irregularity is not available at the moment. As the in situ observation of midlatitude irregularities is quite rare, we hope that the unique and distinctive properties of midlatitude irregularities, at mesoscale and intermediate scale, presented here will stimulate future theoretical study of midlatitude irregularities.