The root-mean-square concentration fluctuation for a passive scalar in a turbulent flow, obtained by integrating the spectrum of Batchelor (1959), can be interpreted in terms of a mixing length times the mean scalar concentration gradient. One contribution to the mixing length is just the length scale of the dominant eddies. For very large Prandtl number there is another contribution which may be interpreted as the distance that a fluid element can be dispersed, according to Taylor's (1921) theory, in the time taken to reduce the minimum scale of the element from the viscous cutoff scale to the diffusive cutoff scale. ¿ American Geophysical Union 1989