Simultaneous density and electric field fluctuation spectra, associated with velocity shear (both in the transverse to the magnetic field ion flow velocity and in the parallel ion flow velocity), have been observed by the DE 2 spacecraft in the F region [Basu et al., 1984>. Basu and Coppi [1988, 1989> argued that the subkilometer scale irregularities are due to collisional electrostatic modes excited in a partially ionized plasma in the presence of shear in the parallel ion flow. In this paper we describe results of a laboratory investigation of the effects of neutral-particle collisions on the parallel velocity shear instability, performed using a double-ended Q machine. The critical value of the shear dv0/dx, the derivative of the field-aligned ion flow with respect to the transverse (to B) coordinate, is determined and compared with theoretical predictions. ¿ 1997 American Geophysical Union