RESULTS are presented concerning the linear stability analysis of an incompressible plane wall jet flow, i.e., flow tangent to a surface and interacting with a still fluid. The numerical solutions of the Orr-Sommerfeld equation are obtained by means of a finite-element approximation. Two different unstable modes are found and two neutral curves are plotted, which show an overlapping region of instability for both types of modes. Experimental flow is realized, and a visualization is compared with a computer-simulated flow field. Contents The stability of the wall jet and the transition of this flow from laminar to turbulent are of interest for different applications. The instability phenomena of the laminar plane wall jet were studied by several authors.15 The temporal linear analysis1'4 gives a second neutral curve within the first; in Ref. 1, this curve is attributed to a higher mode, whereas in Ref. 4 the authors concluded that the inside of the second curve was a stable region. In the present work it is shown that two different modes lead to instability of this flow. A picture of the well-organized structures developing at the onset of transition is also obtained by numerical simulation, and compared with flow visualization. Reducing the Navier-Stokes equation to the Orr-Sommerfeld equation gives L«p = 0 (1) With the fourth-order linear differential operator