The wake behind a horizontally mounted heated cylinder with circular cross-section is visualised using an electrochemical tin precipitation method. In the experiments Re-D and Ri(D), both based on main stream properties, were set to 117 and varied between 0 and 1.5, respectively. The wake becomes 3D from Ri(D) approximate to 0.3. For small Richardson values (RiD < 1), the warm fluid is initially collected in the coherent vortex structures shed from the cylinder. Further downstream thermal plumes originate from these structures. For larger Richardson values (Ri(D) > 1), the upward buoyancy force seems to prevent the formation of the coherent vortex structures and the plumes are already formed close behind the cylinder. The spanwise positions at which the plumes originate, are determined by the occurrence of 3D flow structures at the rear end of the cylinder. The distance between these structures is typically 1.8D and independent of the Richardson number. (C) 2003 Elsevier Science Ltd. All rights reserved.