The quiet north polar cleft at 1400 km has been found to contain many small-scale irregularities in the density of the thermal plasma. Data from the retarding potential analyzer and the cylindrical electrostatic probe on board Isis 2 have shown that these density fluctuations are distributed in a region between 75¿ and 82¿ invariant latitude and approximately dawn to dusk. Their presence is interpreted as a signature of the cleft. The low-latitude edge of this region was not symmetric about noon. The shape and position of the cleft were somewhat influenced by the interplanetary field sector and by season (spring and autumn). Irregularity amplitudes were usually 10% or less, although density changes of a factor of 2 or more have been seen. The typical scale size is 0.7 km. Irregularities were also seen in the superthermal (>20 eV) electrons. However, although the thermal ions and thermal electrons usually fluctuated together, the superthermal electrons fluctuated independently, implying that energy transfer between the superthermal and thermal particles is indirect. Comparison of thermal plasma data with data from other Isis experiments shows that irregularities correlated fairly well with observations of soft particles and auroral optical emissions and not as well with measurements of high-energy particles. This is an additional indication that the energy source for the thermal irregularities is associated with soft particles and that precipitating high-energy particles do not drive the thermal plasma at these altitudes. The position of the cleft determined by plasma irregularities thus agrees best with those cleft descriptions derived from low-energy particle observations.