This experimental study is devoted to the diffusion of a passive scalar downstream from a heated line source located on the centre line of a Benard-von Karman street. Measurements of velocity and temperature have been performed using LDA and cold wire thermometer. The results show that, in the central part of the thermal plume, the transverse heat flux and transverse mean temperature gradient have always the same sign. The failure of the gradient transport model in the case under study is proved to result from the special shape of the probability density function of transverse velocity fluctuations at the source location. (C) 2003 Elsevier Ltd. All rights reserved.