When a fluid-filled rectangular cavity is suddenly heated from one of its sidewalls, the thin boundary layer developing inside the fluid adjacent to this sidewall starts a convective vertical motion. Upon reaching the top wall, this heated flow turns into a horizontal boundary layer which intrudes into the isothermal fluid along the ceiling of the cavity. The shadowgraph technique, which is used to visualize the cavity flow from the bottom, reveals the development of a regular temperature variation in the cross-stream dimension of the intrusion as it travels along the cavity ceiling. These findings are in contrast to the assumption of all previous investigations, that the flow is essentially two-dimensional. The three-dimensional pattern discussed here can be linked to a Rayleigh−Benard instability, which is caused by the fact that the upper part of the intrusion flow is effectively heated from below.