Experiments were performed to confirm the occurrence and growth of longitudinal vortices in a laminar boundary layer developing in water over a heated horizontal flat plate with uniform surface temperature. Photographs of the vortices, measurements of the conditions of their onset, and measurements of their wavelength are presented. Comparisons are made with theoretical instability results for the critical Grashof number and wavelength. Temperature profiles across the boundary layer were measured for flows with and without vortices to show qualitatively the effect that the longitudinal vortices have on the heat transfer rate at the plate. Under conditions of thermal instability the longitudinal vortices were found to be the first stage of the laminar-turbulent transition process in a boundary layer heated from below.